Book A/^i ^^^'^ 

PRESENTED BY 



THE ECOLOGICAL EELATIONS OF ROOTS 



BY 

JOHN E. WEAVER 
Professor of Plant Ecology in the University of Nebraska 




Published by the Carnegie Institution of Washington 
Washington, 1919 



THE ECOLOGICAL EELATIONS OF ROOTS 



BY 



JOHN E. WEAVER ^ 4 4 




Published by the Carnegie Institution of Washington 
Washington, 1919 



CARNEGIE INSTITUTION OF WASHINGTON 
Publication No. 286 



Copfei this Bo0k 
wm'^ first issued 

SEP 5 tS?f 



PRESS OF GIBSON BROTHERS 
WASHINGTON. D. C. 



CONTENTS. 



List of Illustrations . 



PAGE 
V 



Introduction . 



PAGE 

1 



I. The Prairies. 



Panicum virgatum 4 

Andropogon furcatus 4 

Andropogon scoparius 

Andropogon nutans 5 

Stipa spartea 6 

Koeleria cristata 6 

Elymus canadensis 6 

Agropyrum repens 6 

Distichlis spicata 6 

Sporobolus longif olius 7 

Ajistid'a oligantha 8 

Bulbilis dactyloides 8 

Bouteloua gracilis 8 

Liatris punctata 9 

Liatris scariosa 9 

Solidago rigida 9 

Solidago canadensis 11 

Silphium laciniatum 11 

Amorpha canescens 11 

Helianthus rigidus 11 



Brauneria pallida 11 

Petalostemon candidus 11 

Vernonia baldwinii 12 

Kuhnia glutinosa 12 

Verbena stricta 12 

Grindelia squarrosa 12 

Glycyrrhiza lepidota 13 

Astragalus crassicarpus 13 

Psoralea tenuiflora 14 

Psoralea argophylla 14 

Baptisia bracteata 14 

The Suhclimax Prairie 15 

Brauneria pallida 15 

Lygodesmia juncea 16 

Lespedeza capitata 16 

Ceanothus ovatus 17 

Amorpha canescens 17 

Prairie Root Systems and Prairie Environ- 
ment 18 



II. The Chaparral Community. 



Symphoricarpos vulgaris 25 

Rhus glabra 26 

Corylus americana 27 



Vitis vulpina 27 

Rosa arkansana 27 

Shrub Root Systems and the Shrub En- 
vironment 28 



III. The Prairies 

The Root Systems of the Grasses 

Agropyrum spicatum 

Festuca ovina ingrata 

Poa sandbergii 

Koeleria cristata 

The Root Systems of other Prairie Species . 

Lupinus ornatus 

Lupinus leucophyllus 

Astragalus arrectus 

Balsamorhiza sagittata 

Geranium viscosissimum 

Wyethia amplexicaulis 



OF THE Pacific Northwest. 

31 Heuchera glabella 37 

31 Leptotsenia multifida 37 

33 Helianthella douglasii 37 

33 Hoorebekia racemosa 37 

33 Lithospermum ruderale 37 

33 Sieversia ciliata 37 

33 Sidalcea oregana 37 

35 Hieracium scouleri 38 

35 Potentilla blaschkeana 38 

35 Eriogonum heracleoides 38 

35 Prairie Root Systems and Prairie Environ- 

35 ment 38 



IV. The Plains Association. 



Bouteloua gracilis 46 

Aristida purpurea 46 

Muhlenbergia gracillima 47 

Gutierrezia sarothrse 49 

Psoralea tenuiflora 49 

Artemisia frigida 50 

Argemone platyceras 50 

Yucca glauca 51 

Agropyrum glaucum 52 

Carex pennsylvanica 52 

Andropogon scoparius 52 

Stipa comata 53 

Lithospermiun linearifolium 53 



Lygodesmia jimcea 53 

Aragallus lambertii 55 

Petalostemon purpureus 55 

Petalostemon candidus 57 

Eriogonum jamesii 57 

Ratibida columnaris 59 

Senecio aureus oblanceolatus 59 

Asclepias verticillata pumila 59 

Opuntia camanchica 61 

Opuntia fragilis 62 

Plains Root Systems and the Plains En- 
vironment 63 



III 



IV 



CONTENTS. 



V. The Sandhills Subclimax. 



PAGE 

Redfieldia flexuosa 68 

Calamovilf a longifolia 68 

Andropogon hallii 70 

Muhlenbergia pungens 70 

Sporobolus crj^ptandrus 71 

Eriogonum microthecum 71 

Artemisia filifolia 73 

Tradescantia virginiana 73 



PAGE 

Heliotropium convolvulaceum 74 

Petalostemon villosus 74 

Gilia longiflora 75 

Euphorbia petaloidea 76 

Psoralea lanceolata 78 

Ipomoea leptophylla 78 

Sandhill Root Systems and the Sandhill 

Environment 79 



VI. The Gravel-Slide Community. 



Krynitzkia virgata 81 

Paronychia jamesii 82 

Aletes acaulis S3 

Apocynum androssemifolium 84 

Smilacina stellata 85 

Pachylophus csespitosus 85 



Thlaspi alpestre 86 

Mentzelia multiflora 87 

Eriogonum flavum 87 

Gravel-Slide Root Systems and Gravel-Slide 

Environment 88 



VII. The Half-Gravel-Slide Community. 



Elymus triticoides 92 

Solidago oreophila 92 

Rubus deliciosus 93 

Besseya plantaginea 94 

Geraniiim csespitosum 95 

Calamagrostis purpurascens 95 

Koeleria cristata 95 



Giiia aggregata 95 

Potentilla arguta glandulosa 97 

Frasera speciosa 97 

Aster porteri 98 

Half-Gravel-Slide Root Systems and the 

Half-Gravel-Slide Environment ... 98 



VIII. The Forest Community. 



Pu'ola chlorantha 100 

Thalictrum fendleri 100 

Erigeron asper 101 

Erigeron macranthuf 101 

Fragaria virginiana 102 

Allium cernuum 103 

Aralia nudicaulis 103 

Opulaster opulifolius 103 

Ribes lacustre 104 



Rosa acicularis 104 

Arctostaphylos uva-ursi 105 

Senecio cernuus 105 

Castilleia miniata 105 

Heuchera parvif olia 105 

Saxifraga bronchialis 106 

Haplopappus parryi 107 

Forest Root Systems and the Forest 108 
E^ivironment. 



IX. ECADS. 



Smilacina stellata 110 

Chamsenerium angustifolium 110 

Elymus triticoides 112 

Bouteloua gracilis 112 

Stipa comata 113 

Yucca glauca 114 



AUionia linearis 114 

Abronia fragrans 115 

Koeleria cristata 117 

Chrysopsis villosa 117 

Euph orbia montana 119 

Root Modifications of Polydemics 121 



Summary 



X. Summary. 

. . 122 I Bibliography . 



128 



LIST OF ILLUSTRATIONS. 

PLATES. 



Plate A. 

Quadrat-bisect showing root distribution 
of certain dominant and subdominant 
plains species: A, Aristida purpurea; 
B, Bouteloua gracilis; Ar, Artemisia 
frigida; P, Psoralea tenuiflora; C, 
Chrysopsis villosa; Y, Yucca glauca. 

Plate B. 

Quadrat-bisect in the half-gravel-slide. The 
face of the trench was cut along the 
front of quadrat shown in Plate 25, A : 
S, SoKdago oreophila; A, Allium cer- 
nuum; E, Elymus triticoides, fragments 
of which are shown in blue; C, Cala- 
magrostis purpurascens; H, Heuchera 
parvifoUa; B, Besseya plantaginea. 
Plate C. 

Quadrat-bisect showing root systems of 
shrubs and herbs of the forest floor. 
This was made along the front edge 
of the quadrat shown in Plate 27, B : 
H, Haplopappus parryi; R, Rosa acicu- 
laris; Pi, Pinus flexilis; F, Fragaria 
virginiana; C, Chamoenerium, anqvs- 
tifolium; P, Picea engelmanni. 

Plate 1. 

A. One end of a trench used in excavating 

root systems. 

B. Distichlis spicata, showing the long rhi- 

zomes and shallow roots. 

Plate 2. 

A. A prairie of eastern Nebraska in June, 

Erigeron ramosus and Meriolix ser~ 
rulata in the foreground, and bushy 
Psoralea tenuiflora in background. 

B. An area dominated by Agrostis hiemalis, 

with Allium mutabile, Achillea mille- 
folium, and Stipa spariea. 

Plate 3. 

A. Andropogon furcatus. 

B. Andropogon scoparius. 

C. Andropogon nutans. 
Plate 4. 

A. Panicum virgafum, showing rhizomes, 

coarse roots, and complete single root. 

B. Stipa spar tea. 
Plate 5. 

A. Kceleria cristata. 

B. Elymus canadensis. 
Plate 6. 

A. Sporobolus longifoUus, 

B. Bouteloua gracilis, excavated near the 

quadrat shown in text-figure 1. 

Plate 7. 

A. Silphium laciniatum,, roots partially exca- 

vated. 

B. Amorpha canescens, showing the wide 

lateral spread. 

Plate 8. 

A. Kuhnia glutinosa, roots partially exca- 
vated. 



Plate 8 — continued. 

B. Glycyrrhiza lepidota; a' is a continua- 
tion of a. 

Plate 9. 

A. Astragalus crassicarpus, mature root 

system. 

B. Astragalus crassicarpus, showing root of 

young plant. 

C. Baptisia hracteata. 
Plate 10. 

A. Psoralea tenuiflora, the tap root decayed. 

B. Psoralea argophylla, showing entire root 

in center. 
Plate 11. 

A. Brauneria pallida. 

B. Lygodesmia juncea, in two sections. 

C. Lespedeza capitata. 
Plate 12. 

A. Ceanolhus ovatus, root of a thirteen-year 

old plant; a' is a continuation of o. 

B. Amorpha canescens; a' is a continuation 

of a. 
Plate 13. 

A. Symphoricarpos vulgaris, showing fine 

network in surface soil. 

B. Rhizomes and runners of Symphoricarpos 

vulgaris; the horizontal line is the 
ground line. 
Plate 14. 

A. Rhus glabra, a portion of the root network 

with ascending rootlet a. 

B. Rhus glabra, mth ascending rootlets. 
Plate 15. 

A. Corylus americana, the roots shown in 

two sections. 

B. Rosa arkansana, the roots shown in two 

sections. 

C. Corylus americana, rhizomes and roots 
Plate 16. 

A. Rosa arkansana, showing method of 

propagation. 

B. Rhus glabra invading subclimax prairie. 
Plate 17, 

A. Yucca glauca, showing the multicipital 

stems and rhizome habit. 

B. Prairie of southeastern Washington. 

C. Meter quadrat in the prairie, showing 

Balsamorhiza, Festuca, Lithospermum, 
and Hieracium. 
Plate 18. 

A. The plains association near Colorado 

Springs, showing Aristida purpurea 
bunches in Bouteloua gracilis turf. 

B. Psoralea tenuiflora, in two sections. 

C. Yucca glauca. 
Plate 19. 

A. Stipa comata. 

B. Petalostemon Candidas. 

C. Eriogonum jamesii. 
Plate 20. 

A. Short-grass plains, showing Bouteloua 

gracilis and Opuntia polyacantha. 

B. General view of the sandhill community. 



V 



VI 



LIST OF ILLUSTRATIONS. 



Plate 21. 

A. A sandhill community, showing Red- 

fieldia, Petalostemon villosus, Psoralea 
lanceolata, and Chrysopsis villosa, with 
a socies of Eriogonum microthecum 
at the left. 

B. Roots and rhizomes of Calamovilfa 

longifolia. 
Plate 22. 

A. Andropogon hallii. 

B. Muhlenhergia pungens. 
Plate 23. 

A. Consocies of Aletes acaulis on the gravel- 

slide, with Krynitzkia virgata in the 
foreground. 

B. Quadrat on the gravel-slide, showing 

detail of surface. 
Plate 24. 

A. Aletes acaulis. 

B. Network of fine rootlets of Aletes. 

C. Smilacina stellata. 

D. Thlaspi alpestre. 
Plate 25. 

A. Half-gravel-slide, showing Elymus trit- 
icoides and the large bare intervals. 



Plate 25 — continued. 

B. Elymus triticoides. 

C. Rubus deliciosus. 
Plate 26. 

A. Koderia cristata. 

B. Calamagrostis purpurascens. 

C. Potentilla arguta glandulosa. 
Plate 27. 

A. Picea engelmanni consociation, showing 

the forest floor. 

B. Quadrat in the same spruce forest, show- 

ing Haplopappus parryi, Fragaria 
virginiana, Thalictrum fendleri, etc. 
Plate 28. 

A. Arctostaphylos uva-ursi, showing a por- 

tion of the root system. 

B. Ribes lacustre, seven years old. 
Plate 29. 

A. Root systems of ecada of Smilacina stel- 

lata; a, gravel-slide; 6, spruce forest. 

B. Bouteloua gracilis from the sandhills. 

C. Abronia fragrans from the plains. 
Plate 30. 

A. Opulaster opulifolius, twelve years old. 

B. Chrysopsis villosa from the sandhills. 



TEXT-FIGURES. 



PAGE 

1. A meter quadrat in the Bouteloua com- 
munity dominating a gravelly 
ridge at Belmont, near Lincoln; B, 
Bouteloua gracilis; A, Andropogon 
furcatus; S, Sporobolus longifolius; 
M, Meriolix serrulata; Am, Amor- 



pha canescens 9 

2. Root system of Liatris punctata 10 

3. Root system of Kuhnia glutinosa .... 10 

4. Graphs showing the average daily 

temperature (heavy line) and 
humidity (light line) in the 
prairie during 1916 22 

5. Graphs showing the average daily 

evaporation rates in the prairie 
during 1916 (heavy line) and 1917 
(light line) 22 

6. Schematic bisect showing the root and 



stem relations of important 
prairie plants. This and figures 
7 and 8 were drawn from photo- 
graphs and data obtained by the 
excavation and examination of 
325 root systems of these 18 
species: H, Hieracium scouleri; 
K, Koeleria cristata; B, Balsam- 
orhiza sagittata; F, Festuca ovina 
ingrata; G, Geranium viscosis- 
simum; P, Poa sandbergii; Ho, 
Hoorebekia racemosa; Po, Poten- 
tilla blaschkeana 32 

7. Schematic bisect: S, Sieversia ciliata; 
W, Wyethia amplexicaulis; LI, 
Lupinus leucophyllus; Lo, Lupi- 
nus ornatu^; P, Poa sandbergii; 
E, Leptotoenia muUifida; A, 
Agropyrum spicatum 34 



PAGE 

8. Schematic bisect: Hu, Heuchera gla- 
bella; A, Astragalus arrectus; S, 
Sidalcea oregana; H, Helianthella 
douglasii; Ag, Agropyrum spica- 



tum 36 

9. Aristida purpurea 47 

10. Artemisia frigida 47 

11. Gutierrezia sarothrce, showing wide 

lateral spread of roots in the 
surface soil 48 

12. A small part of caudex of Yucca glauca, 

showing number and extent of 
horizontal roots, many exceeding 
25 to 30 feet in length 48 

13. Argemone platyceras 54 

14. Lithospermum linear if olium 54 

15. Lygodesmia juncea 56 

16. Aragallus lambertii 56 

17. Petalostemon purpureus 58 

18. Ratibida columnaris 59 

19. Senecio aureus oblanceolatus 60 

20. Underground parts of Asclepias verti- 

cillata pumila 61 

21. Opuntia camanchica, showing vertical 

anchorage roots 62 

22. Top view of surface roots of Opuntia 

camanchica, showing the wide 
lateral spread 62 

23. Redfieldia flexuosa, showing rhizomes 

and root distribution 69 

24. Eriogonum microthecum 72 

25. Root system of a ten-year-old Arte- 

misia fiUfolia 74 



LIST OF ILLUSTRATIONS. 



VII 



PAGE 



26. Tradescantia virginiana 74 

27. Root system of Petalostemon villosus . . 75 

28. Gilia longiflora 76 

29. Underground parts of Psoralea lanceo- 

lata, showing root tubercles at a 
depth of eight feet 77 

30. Ipomoea leptophylla, showing a portion 

of the very extensive root system 78 

31. Krynitzkia virgata. The heavy slanting 

line indicates the actual ground- 
line 81 

32. Paronychia jamesii 82 

33. Surface view of a single root of Paro- 

nychia jamesii at a depth of 2 to 
2.5 inches 83 

34. Apocynum androscemifolium, showing 

rhizomes and dense network of 
roots 84 

35. Pachylophus coespitosus 86 

36. Eriogonum flavum 87 

37. Quadrat-bisect showing root relations 

of gravel-slide plants. This was 
made along the front of quadrat 
shown in Plate 23, B : P, Paro- 
nychia jamesii; A, Aletes acaulis; 
K, Krynitzkia virgata 90 

38. Solidago oreophila, showing wide- 

spreading lateral and deep cen- 
tral roots 93 



PAGE 



39. Besseya plantaginea, showing the wide- 

spreading root system 94 

40. Root system of Geranium coespitosum. 96 

41. Root system oi Frasera speciosa 97 

42. Pirola chlorantha 100 

43. Thalicirum fendleri 101 

44. Rhizome and roots of Erigeron asper. . 102 

45. Fragaria virginiana 102 

46. Allium cernuum 102 

47. Rhizomes and root system of Aralia 

nudicaulis 103 

48. Senecio cernuus 106 

49. Heuchera parvij'olia 107 

50. Haplopappus parryi 107 

51. Root systems of ecads of Chamoe- 

nerium angusti folium: a, gravel- 
slide; b, forest Ill 

52. Fragments of the roots of Yucca 

glauca: a, plains form; b, half- 
gravel-slide form 114 

53. Allionia linearis from the plains 115 

54. Allionia linearis from the sandhills. . . 115 

55. Abronia fragrans from the sandhills. . 116 

56. Chrysopsis villosa, showing various 

stages of development in the 

plains soU 118 ^ 

57. Euphorbia montana from the plains . . . 120 

58. Euphorbia montana from the half- 

gravel-slide 120 



THE ECOLOGICAL EELATIONS OF EOOTS. 



INTRODUCTION. 

Although considerable progress has been made in the study of root 
systems of desert plants (Cannon, 1911, 1913; Markle, 1917), very 
little information is available concerning the roots of other native 
species. During 1914 the writer investigated the root systems of the 
prairie plants of southeastern Washington, where the annual pre- 
cipitation is only 21.6 inches and occurs mostly in the period of rest. 
Since that time it has been planned to make a comparative study of 
the roots of prairie plants growing in a more humid region and where 
the precipitation occurs mostly during the season of plant growth. 
The opportunity for such study came during the fall of 1917 and work 
was carried on vigorously until the soil became frozen in December. 
The study was resumed early in the following spring. In June 1918 
the field of investigation was extended to the Great Plains and sand- 
hill region of Colorado, while later in the summer a large number of 
plants were excavated and examined in various habitats about Pike's 
Peak in the Rocky Mountains. 

A knowledge of root distribution and root competition under dif- 
ferent natural conditions is not only of much scientific value, but it 
also finds practical application in a better understanding of the value 
of plants as indicators for distinguishing lands of grazing value only 
from those with possibilities of crop production. It will result in a 
more intelligent solution of the ecological problems of grazing and will 
likewise be of great aid to the forester in selecting sites for afforesta- 
tion. Moreover, a knowledge of root distribution will throw a flood 
of light upon many of the problems of plant succession. Indeed, the 
phenomena of ecesis, competition, and reaction can not be completely, 
if indeed correctly, interpreted without a knowledge of the extent, 
position, and relation of the root systems of the plants. 

Shantz has given us an excellent example of the value of a knowl- 
edge of root distribution in his study of the natural vegetation of the 
Great Plains as an indicator of the capabilities of land for crop produc- 
tion (1911). Sampson (1914, 1917) has made a study of root systems 
of many range plants in considering their life history, forage value, 
and the natural re vegetation of range lands; while foresters are just 
beginning to study the roots of plants as indicators of conditions of 
soil moisture on various sites (Korstian, 1917). 

This paper contains descriptions of the character, depth, and dis- 
tribution of the roots of about 140 species of plants. These include 
shrubs, grasses, and other herbs, a few of the latter being noxious 

1 



2 



THE ECOLOGICAL RELATIONS OF ROOTS. 



weeds. It represents the results of the examination of approximately 
1,150 individual plants in 8 different communities, as follows: prairies 
of eastern Nebraska, chaparral of southeastern Nebraska, prairies of 
southeastern Washington and adjacent Idaho, plains and sandhills of 
Colorado, the gravel-slide, the half-gravel-slide, and forest communi- 
ties of the Rocky Mountains of Colorado. 

The method employed in excavating root systems was to dig trenches 
2 to 3 feet wide and 6 to 10 feet long to a depth of about 6 feet by the 
side of the plants to be examined. This offered an open face into which 
one might dig with a hand pick furnished with a cutting edge on one 
end, and, after sufficient practice and acquaintance with the soil 
texture, successfully excavate a root system almost in its entirety. 
Of course, the trenches were deepened as work progressed and the 
working level sometimes reached a depth of 10 or even 16 feet (plate 
1). A total of more than 100 such pits was used. To assure absolute 
certainty as to maximum depth, for extreme care must be used in 
excavating root termini, the soil underlying the deepest roots was 
usually undercut about 12 to 18 inches below the root-ends and care- 
fully examined as it was removed. For the deepest-rooted species of 
herbs and shrubs it was found expedient to have a longer trench with 
two levels, one at about 7 to 9 feet and a second one about twice as 
deep. Thus the soil could be removed from the lower to the higher 
level as work proceeded. Indeed, in several cases where the roots 
extended to depths of 18 or 20 feet or more (for example Rosa arkan- 
sana or Lygodesmia juncea)j the deeper soil was removed by means of 
a bucket attached to a rope. Considerable danger from caving was 
experienced, especially in the sandhill soils and also in the deeper loose 
loess soils as well. In fact, it was found inexpedient to remove the 
entire root system in a few cases. In each community the work 
extended over a field sufficiently large, sometimes several square 
miles in extent, so that any local differences in soil texture, etc., were 
eliminated. 

All of the roots examined, except as otherwise indicated, were of 
mature perennial plants. The practice followed was to examine 
several roots of a given species and then to write a working descrip- 
tion of the root system. These descriptions were kept at hand, and as 
new roots of the same species were studied, any variation from the 
original description was carefully noted. While many of the root 
systems, especially those of the grasses, were removed in their entirety 
and photographed against an appropriate background, and a few 
photographed in position, many others were drawn in place. The 
sketching was first done with pencil on a large drawing-sheet ruled 
to scale. Drawings were made simultaneously with the excavating of 
the root and always to exact measurements. When entirely com- 
pleted they were retraced with India ink. Such a drawing often repre- 



INTRODUCTION. 



3 



sents the extent, position, and minute branching of the root system 
more accurately than a photograph. This is especially true of the more 
extensive root systems, for here, even under the most favorable con- 
ditions, the photograph is always made at the expense of detail, many 
of the finer branches and root-endings being obscured. 

During the course of these investigations the writer has become 
greatly indebted to the following persons for faithful assistance in the 
pursuance of this work. It is a pleasure to acknowledge here the 
indebtedness to my students, Messrs. F. C. Jean and Alvin Goke, for 
much help in the excavation of the plants, and to Miss Annie Mogen- 
sen and Mrs. F. C. Jean for their assistance in drawing many of the 
root systems. I wish also to acknowledge the helpful suggestions and 
encouragement given by Dr. F. E. Clements and Dr. R. J. Pool 
throughout the period of the work. To Professor T. J. Fitzpatrick 
I am also indebted for careful reading of the manuscript and proof. 



4 



THE ECOLOGICAL RELATIONS OF ROOTS. 



I. THE PRAIRIES.i 

The prairies of eastern Nebraska in which these studies were made 
are too well known to need discussion here (plate 2) . A general floristic 
description has been given by Pound and Clements (1898, 1900), a 
more detailed one for a portion of southeastern Nebraska by Thornber 
(1901), and recently a more specific account of the local region under 
consideration by Weaver and Theil (1917). As the various species are 
considered, brief comment will be made upon their relative importance. 
Therefore, we nrny proceed to a consideration of the root habits of the 
individual species and then examine the environmental conditions 
under which the pknts grow. Finally, the correlations between root 
habit and environment will be discussed. This sequence will be fol- 
lowed for each plant community. 

Panicum virgatum. — ^Although showing a preference for loose sandy soils, 
panic grass grows abundantly in many situations throughout the prairies. 
Its size, abundance, and duration combine to make it an important com- 
ponent of the prairie vegetation, where it often holds the rank of a dominant 
species. 

This grass has the longest root system of any species examined. The roots 
are very coarse, many having a diameter of 3 or 4 mm. They pursue a verti- 
cally downward course, spreading only a little near the surface, to a maximum 
depth of over 9 feet. Several plants reached depths of over 8.5 feet (plate 
4, a). In the first 6 or 7 feet of soil the roots are very little branched, the 
laterals (usually less than 3 inches long) occurring only scatteringly. In the 
deeper soil numerous fine laterals occur, although the last 6 to 12 inches are 
often very poorly branched and the roots end abruptly. The main roots 
remain nearly uniform in diameter for most of their course or taper so slowly 
that at 5 or 6 feet they may still have a diameter of 2 mm. The type of soil 
considerably modifies not only the root penetration but also the manner of 
branching. The plants that were growing in a gravelly soil with a sandy 
subsoil underlaid with an impervious blue clay were found to penetrate only 
7 feet to the clay, but the branching was much more pronounced. The cortex 
on younger roots is pearly white and very brittle; on older ones smooth, 
pinkish white, and of a papery consistency. Eight plants were examined. 

Andropogon furcatus. — The two bluestems are dominants among prairie 
grasses. They form a large part of the prairie hay of Kansas and Nebraska. 
The taller and deeper but coarser-rooted A. /wrcaiws is less resistant to drought 
than the shorter, finer-rooted A. scoparius, as is shown both by their local 
and general distribution. Throughout the prairies the former luxuriates in 
the draws and on lower lands, while the latter dominates higher areas. West- 
ward the big bluestem soon drops out, while little bluestem not only forms a 
"dominant of widest distribution and most controlling influence in the bunch- 
grass association of the sandhills" (Pool, 1914:224) and '^enters into dis- 
turbed areas of the wiregrass association in Colorado" (Shantz, 1911:52), 
but also occurs on rough hillsides where water penetrates readily, throughout 
much of the area included in Wyoming and Montana. 

^ The major portion of the work in this section was done by the writer in conjunction with Mr. 
Lyman H. Andrews, who voluntarily joined the colors in December 1917, and to whom joint 
credit is herewith acknowledged. 



THE PRAIRIES. 



5 



Ten Eyck (1904:216) examined the roots of A. furcatus at Manhattan, 
Kansas, and found that they "form a dense, tough sod, from 6 to 8 inches 
thick, and the subsoil is filled with a great mass of roots." "The roots were 
broken off at 4.5 feet from the surface, but from their size they must have 
extended at least 2 feet deeper into the compact clay subsoil." 

Twelve plants were examined near Lincoln. The very abundant roots 
grow both vertically and obliquely downward, a few almost horizontally, and 
at once thoroughly occupy the soil and form a dense sod. The roots may 
extend obliquely away from the bunches to more than a foot before turning 
downward. The larger roots vary from 0.5 to 3 mm. in diameter and may 
reach a depth of 6 feet and 10 inches. Most of the plants examined reached 
depths of over 6 feet (plate 3, a). In locations where a hard clay subsoil 
occurred the roots were 2 to 2.5 feet shorter. All of the roots branch profusely, 
the main laterals being from 2 to 6 inches long. However, here again the 
amount of branching and the length of the laterals are closely correlated with 
the soil texture, always being less in hard soils. The roots taper so gradually 
that at 4 feet thay are nearly as large as at the surface. The soil is thoroughly 
occupied to a depth of 5 feet. The ends of the roots are extremely well 
branched to the very tip. In color the roots are reddish-brown. They have 
a very loose papery cortex which is easily removed and reveals the tough, 
yellowish stele. 

Andropogon scoparius. — This grass is figured by Shantz (1911:56) as 
extending to a depth of about 5 feet and thoroughly occupying the sandy soil 
in the bunchgrass association of eastern Colorado. It is interesting to note 
that the deep-rooted Panicum virgatum also occurs here. 

In these studies plants of this species were examined in two soil tjrpes. The 
first group was in porous, gravelly soil mixed with sand and underlaid with a 
rocky subsoil of decayed sandstone at a depth of 3 feet. The other group 
grew in clay-loam soil with a clay subsoil. In the former habitat none of the 
roots of the several plants examined reached depths greater than 28 inches, 
while in the clay loam several plants had a maximum root depth of about 65 
inches (plate 3, b). The roots are much finer than those of A. furcatus, being 
only 0.1 to 0.8 mm. in diameter. The lateral spread of the roots is very 
similar but somewhat more pronounced. Roots are so abundant as to form 
a dense sod, completely filling the soil to a depth of from 12 inches in gravelly 
soil and to as much as 30 inches in clay loam. The surface is especially well 
occupied with dense masses of finely branched rootlets. All the roots branch 
profusely to the third or fourth order, many of the branches being over 30 
inches long. The deeper soil (from 30 to 60 inches) is fairly well occupied. 
The roots are light-brown in color and have a very thick cortex which peels 
off easily, thus exposing the tough yellowish stele. The deeper roots are 
lighter in color, profusely branched, and very brittle. 

Andropogon nutans. — Goldstem is a dominant in the subclimax prairie. 
Like the other andropogons and Panicum virgatum, it also matures late in 
summer. It is one of the deeper-rooted prairie grasses. Of the 5 plants 
examined the maximum depth of root varied between 51 and 59 inches 
(plate 3, c). These were growing in a clay-loam soil which extended to a 
depth of 3.5 feet and below which occurred pure sand. The roots vary from 
2 mm. to less than 0.5 mm. in diameter. They are very abundant, spread 
laterally but little, and completely occupy the soil, branching profusely to the 
second and third order. Within a foot from the surface, however, most of 
the roots become less than 0.5 mm. in diameter, forming a dense network to 
a depth of about 3 feet. Even in the fourth foot the roots are quite numerous. 



6 



THE ECOLOGICAL RELATIONS OF ROOTS. 



many of them breaking up at this depth into clusters of small branches. The 
roots are slightly reddish in color. The branching is characteristic but difficult 
to describe. 

Stipa spartea. — This perennial grass is one of the dominants in the prairies, 
being especially conspicuous during the month of June, when it gives tone to 
the estival aspect. Its root system is rather meager when compared with 
most of the other prairie grasses. Strong fibrous roots from 1 to 1.5 mm. in 
diameter descend rather vertically into the soil to a maximum depth of only 
21 to 26 inches (plate 4, b). A few spread laterally in a diagonal direction to 
a distance as great as 10 or 12 inches. Numerous smaller roots fill the surface 
soil, while the larger ones send off many laterals to a depth of about 14 inches, 
where the main roots may break up into many fine branches. The hairy 
cortex causes the soil to cHng to it very closely. This pulpy cortex has a 
grayish-white color and peels off very easily, leaving a very wiry, tough stele . 

Koeleria cristata. — ^Whether on the prairies of Minnesota or Nebraska or 
in the Pacific Northwest, Koeleria is an important grass in the estival aspect. 
As in the case of Stipa spartea, which also makes a rapid growth and blossoms 
early, the vegetative and reproductive activity may be correlated with the 
shallow root system. In eastern Washington, where Koeleria is a common 
bunchgrass, it flowers in late June, soon dries up, and remains dormant until 
revived by the autumn rains. In this region the deepest root found was at 
28 inches, and 15 inches was determined as the average depth (Weaver, 1915). 

This plant has a very shallow but exceedingly well-developed root system. 
None of the roots of the 7 plants examined reached depths of over 21 inches, 
while the average root depth was about 15 inches (plate 5, a). A great abun- 
dance of fine rootlets spread out from the base of the plant and occupy the 
soil exclusively for a distance of 8 inches on each side of it. Indeed, some of 
the roots run almost horizontally and are less than 0.5 inch deep in the soil. 
The roots are yellowish-brown in color, usually less than 0.2 mm. in diameter, 
and branch and rebranch to form a dense mat. 

Elymus canadensis. — This plant is an important prairie species of wide 
distribution. Like Koeleria, it is comparatively shallow-rooted, but has a 
widely spreading root system. Five plants in loess soil gave maximum root 
depths of 16, 17, 20, 21, and 22 inches respectively. Plate 5, b, shows the 
rather meager root development when compared with most of the other 
grasses. The obliquely running roots reach distances of 20 inches or more 
on either side of the base of the plant. Lateral rootlets are seldom over 2 
inches long and may branch to the third or fourth order. The roots are white 
in color and from 0.1 to 0.5 mm. in diameter. They are very tough and wiry. 

Agropyrum repens. — From the extensive rhizomes of this perennial, which 
are often 2 to 3 feet long, arise numerous fine, silvery-white roots. These 
send off rather poorly branched laterals as they descend somewhat vertically- 
downward, some to a maximum depth of 8 feet. The first 30 inches of soil 
is abundantly filled with roots, while many penetrate to a distance of 4 to 7 
feet. Many of the roots of the 10 plants examined pursued a peculiar zigzag 
course, a character which was more pronounced in the deeper roots. 

Distichlis spicata. — This low, dioecious perennial of seacoasts and alkaline 
soils is still quite abundant on the ''salt flats'' about Lincoln, although it is 
being replaced by other species as the soil becomes less salty, due to better 
drainage resulting from the straightening of stream courses. It has consider- 
able forage value throughout the West, where it occurs abundantly in low 
saline situations. About Lincoln most of the area occupied by salt grass is 



THE PRAIRIES. 



7 



alluvial wash, the various soil strata often showing quite distinctly (plate 
1, b). The plants examined were growing in pure stands. A chemical exami- 
nation of the first 8 inches of soil (the samples being taken soon after a rain 
had wet the soil to just this depth) gave a salt content of 2.6 per cent. This 
is less than in the adjacent and successionally earlier Atriplex zone, which 
gave 3.1 per cent alkaUnity. In both cases the greater part of the salt was 
sodium carbonate.^ 

In the succession it is replaced by Agropyrum repens, which in turn is fol- 
lowed by Sporoholus longifoUus, Panicum virgatum, Andropogon furcatus, 
and other prairie plants. An interesting successional sequence was deter- 
mined in a rich alluvial flood-plain, where the water-level occurred in gumbo 
soil at a depth of about 6 feet. Although only an occasional specimen of 
Distichlis was to be seen, the soil contained three distinct strata of the abun- 
dant and well-preserved rhizomes at depths of 13, 9, and 6 inches respectively. 
These indicated successive overflows and deposits. Above these the soil was 
filled with a dense network of the rhizomes of Agropyrum repens. However, 
only a few of these plants were still alive, the soil being almost completely 
occupied by alternate areas of Sporoholus longifoUus and Bulbilis dactyloides. 

Distichlis has better developed rhizomes than almost any other grass 
examined. They vary from 2 to 5 mm. in diameter, often being somewhat 
flattened parallel with the soil surface. One specimen was found to be 9 feet 
long. It supported 19 tufts of plants. The rhizomes are exceedingly tough, 
being covered with a hard, shell-like cortex. The terminal buds are long and 
sharp-pointed, as are also the leaf-scales at the regularly spaced nodes which 
are about 2 inches apart. The rhizome depth is quite uniform at from 4 to 6 
inches. The rhizomes branch extensively, sending off laterals in all directions. 

The roots are relatively shallow, but few occur in the first 4 inches of soil. 
They branch from the rhizomes in both vertical and horizontal directions, 
and while the lateral spread is not great, they occupy the soil thoroughly to 
a depth of about 18 inches. Relatively few reach a depth of over 2 feet. 
They are from 1 to 3 mm. in diameter and branch rather sparingly to the 
third order, these laterals being only 1 or 2 inches long. The deeper roots are 
even more sparingly furnished with laterals, which are only 1 to 3 mm. in 
length. The yellowish-white cortex of the older roots is thick and pulpy. 
When removed it reveals a pearly-white stele. Thirteen plants were examined. 

Sporobolus longifolius. — This perennial, late-maturing grass is often an 
important component of prairie vegetation, especially in the earlier stages of 
development. Although it occurs in typical prairie, it is often more abundant 
on eroding banks along roadsides and in disturbed areas generally from dry 
hill-tops to alluvial flood-plains. The short, thick rootstocks spread in all 
directions and form bunches varying from 4 to 18 inches in diameter. 

The rather coarse, fibrous roots penetrate the soil to an average maximum 
depth of only 24 inches. The greatest root depth of the 17 plants examined 
ranged from 17 to 40 inches. However, the roots are very dense and 
thoroughly occupy all of the soil, spreading laterally from the base of the plant 
in an almost horizontal direction to a distance of from 12 to 20 inches (plate 
6, a). Thus an area of soil of 6 or 7 square feet may be thoroughly occupied 
to a depth of 18 inches by the roots of a single bunch. The roots vary from 
1 to 2 mm. or less in diameter, many of them keeping the original diameter 
to a depth of 18 inches. They are pearl-white in color, very tough and wiry, 
and the cortex is densely covered with short hairs. When the cortex is 
removed it reveals the thick, white stele. All of the roots branch profusely. 



^ The writer is indebted to Professor C. J. Frankforter for these determinations. 



8 



THE ECOLOGICAL RELATIONS OF ROOTS. 



but the laterals are only 0.5 to 2 inches long. These often branch again. The 
roots below the 18-inch level are relatively few and the soil is not well occupied. 
Roots were examined in 3 different soil types and found to be very similar in all. 

Aristida oligantha. — This species is abundant on prairies, especially in drier 
soils. It frequently dominates areas where the soil is shallow, or plays the 
role of an interstitial among the taller grasses. The root systems of 10 plants 
of this annual grass were examined. They were growing in a clay-loam soil. 
The white, threadlike roots are less than 0.2 mm. in diameter and are found 
abundantly only in the first 10 inches of soil. While some of the longest 
reach depths of 40 inches, relatively few occur below 18 inches. They are 
so fine and brittle that it proved impossible to secure material for a photo- 
graph. Compared with other grasses the roots are much less abundant. 

Bulbilis dactyloides. — Perhaps no grass of the western plains, except 
grama, is better known for its valuable characteristics as a pasture grass than 
the buffalo grass. In the region of this study it can not compete with the 
taller prairie plants. However, it is found in dense patches resulting from its 
method of propagating by stolons, on low-lying lands where water may stand 
for a time in ''pockets" or where over-grazing has killed the taller grasses. 

Ten Eyck (1904), working at Manhattan, Kansas, and in a soil of which the 
surface foot was a mellow, dark loam, underlaid with a rather compact, clayey 
subsoil, found "the roots are numerous but they do not penetrate deeply 
into the soil". Shantz (1911: 38), dealing with the hard soils of the Great 
Plains region of Colorado, where "the soil is only rarely wet down to a foot 
or more," states that "almost the entire root system of the short grasses 
{Bulbilis dactyloides and Bouteloua oligostachya) is limited to the first 18 inches 
of soil. They have a very extensive surface root system." Robbins (1917: 
70) states that roots of buffalo grass sometimes go to a depth of 7 feet. This 
latter statement checks up more nearly with our findings. 

A dozen root systems of this grass were examined in two different areas, 
both of which were in alluvial soil on bottom lands. The roots are very fine, 
the largest being less than 0.5 mm. in diameter. They scarcely spread at all 
laterally, but form a dense mat to a depth of 12 to 18 inches, branching mostly 
only to the second order. These hairlike laterals are usually not over 0.5 
inch in length. Although these roots are very fine, they are quite tough and 
easy to follow. At 18 inches depth they become less numerous and are poorly 
branched, so that the deeper soils are sparsely occupied. However, many 
roots occurred at 4.5 feet, and numerous others continued vertically downward 
to a maximmn depth of from 58 to 73 inches and in one of the trenches in 
gumbo soil to the ground-water level. The shallower roots are light-brown 
in color, while the deeper hairhke roots are nearly white. 

Bouteloua gracilis. — Grama grass occurs often in fairly pure stands on the 
lighter soils of gravelly ridges and it is also frequently found dominating 
areas of alluvial soil on bottom lands. Figure 1 shows a typical area domi- 
nated by Bouteloua into which Andropogon furcatus is invading. The soil is a 
very porous coarse sandy to gravelly loam deposited by glacial action, with a 
water-holding capacity of about 40 per cent of its dry weight. This grama 
consocies, in which a number of root systems were examined, dominates a 
ridge at Belmont near Lincoln, which is entirely smTounded by tall prairie 
grasses rooted in clay loam. The transition from one soil type to the other is 
very abrupt and the ecotone is correspondingly sharp. Frequent determina- 
tions invariably showed a higher available water-content in the clay-loam soil. 

The root system is extremely well developed, great masses of fine roots 
occupying every cubic centimeter of soil to a depth of 18 inches. A few roots 



THE PRAIRIES. 



9 



reached a maximum depth of 46 inches, although below 2 feet the roots are 
very sparse (plate 6, b). The largest roots are only 0.5 mm. in diameter. The 
laterals are usually not over an inch long. These abundant threadUke laterals 
branch only poorly. The roots are brownish white in color and of a very firm 
textiue. Other groups of plants examined in two locations on alluvial soils 
showed a somewhat poorer development of the root system, but the general 
distribution and depth were very similar to those growing in the gravelly soil. 
This root distribution stands quite in 
contrast to that found by Shantz in Col- 
orado. This would seem to be a clear 
case of environmental conditions pro- 
foundly modif>dng root development. 

Liatrls punctata. — Of the numerous 
blazing-stars which add beauty to the 
prairies in autumn, L. punctata and L. 
scariosa are probably the most abundant. 
The xeroph>i}ic nature of the former and 
its extensive range far into the drier 
regions westward may be explained in 
part by its deep root system. 

Seven individuals of this species were 
excavated. All had strong tap-roots. 
Those growing in clay with a sandy sub- 
soil reached depths of 11 feet 5 inches 
and 15 feet 9 inches respectively, while 
three growing in clay-loam with a clay 
subso3 penetrated only to a depth of 
about 6 feet 8 inches. A diagram of one 
of the larger specimens (fig. 2), recon- 
structed from a photograph and from a 
drawing made to scale in the field, 
pictiu'es the roots as occurring in one 
plane. On other specimens small laterals were very scarce to a depth of 9 
feet, while at 12 feet the soil was filled with small, silvery white, sparsely 
branched rootlets. This scarcity of absorbing laterals was again noted in the 
clay-loam soil of the high prairie until a depth of about 6 feet was reached. 
Even here the branching was not so pronounced as on the specimens in the 
lighter soil. The older roots are woody and chocolate-brown in color. 

Liatrls scariosa. — This plant, unhke L. punctata with its strong deep tap- 
root, is characterized by a large woody corm, 3 to 5 inches in diameter, from 
which arise very numerous fibrous roots. These are only 1 to 2 mm. in diam- 
eter, but they are very finely branched and spread laterally, thus thoroughly 
occup>dng the first 2 feet of soil. The depth of penetration was not deter- 
mined. 

Solidago rigida. — This goldenrod is not only abundant in the less xero- 
phytic prairies of Minnesota and Nebraska, but extends far westward into the 
drier grasslands. In Wyoming and Montana its presence, together with 
certain other species, invariably indicates rather permeable soils with at least 
a fair amount of available water.^ 

^ During the summer of 1917 the writer, while working on grazing problems in the prairies of 
the Northwest, had an excellent opportunity to trace the westward extension of many species 
typical of the less xerophytic prairies of eastern Nebraska and Minnesota far into the Great 
Plains. Indeed, of the two associations of the prairie-plains climax of Clements (1916), 
the Stipa-Agropyrum prairie occupies much more territory in the Northwest than the Bulhilis- 
Bouteloua plains. 




Fig. 1. — A meter quadrat in the 5owte- 
loua community dominating a 
gravelly ridge at Belmont, 
near Lincoln. 

B, Bouteloua gracilis. 
A, Andropogon furcatus. 
S, Sporoholus longifoUus. 
M, Meriolix serrulata. 
Am, AmoT'pha canescens. 



10 THE ECOLOGICAL RELATIONS OF ROOTS. 

This plant has a root system which spreads immediately below the surface. 
Many of the numerous small roots pursue an oblique direction and spread 
from 12 to 18 inches on either side of the plant before they turn downward. 
Most of the roots are only about 1 mm. in diameter and poorly branched. 




FiQ. 2. — ^Root system of Liatris punctata. Fig. 3. — Root system oiKuhniaglutinosa. 



However, they are very abundant in the first 2 feet of soil, while maximum 
depths of over 5 feet are attained. They are yellowish-brown in color and have 
a smooth cortex, except the lower roots, which are silvery white. Only 4 
plants were examined. 



THE PRAIRIES. 



11 



Solidago canadensis. — ^This abundant prairie goldenrod propagates by 
means of strong woody rhizomes, a centimeter or less in diameter, which run 
horizontally about 2 inches below the soil surface. As in S. rigida, the roots 
are fibrous. They are about 1 to 2 mm. in diameter and pale yellow in color. 
They descend rather vertically into the soil. From these abundant roots 
numerous laterals are sent off, beginning just below the surface and con- 
tinuing to a depth of from 9 to 10 feet. However, the main roots keep their 
identity throughout their course and are very poorly branched throughout 
the last foot. Thus the soil was well occupied with the roots of this species 
and few other roots were found in the Solidago area. Eleven plants were 
examined, most of which gave a maximum root depth of 10.5 feet, while one 
reached 11 feet. 

Silphium laciniatum. — ^The wide distribution of this coarse perennial 
prairie plant is well known. It has a large, fleshy, deep tap-root. The root 
diameter just below the multicipital crown, which gives off numerous shoots, 
varied from 1 to 2 inches in the 5 plants examined. The tap-root descends 
vertically and tapers so rapidly that at 3 feet it may be only 0.5 inch in 
diameter (plate 7, a). It pursues this general vertical direction to a maximum 
depth of from 9 feet to 13 feet 8 inches, the last 2 or 3 feet of its course being 
somewhat tortuous. The whole root is a dark reddish-brown in color, the 
cortex of the first 18 inches being distinctly ridged and wrinkled. Scattered 
laterals, 2 mm. or less in diameter and from 6 to 18 inches long, are present 
on this upper portion of the root. There are relatively few large laterals. 
These frequently run off horizontally for a distance of 3 or 4 feet and then 
turn downward. The deeper roots are also fleshy and brittle and end rather 
abruptly; usually they are freely branched. 

Amorpha canescens. — This half-shrub is a characteristic subdominant of 
the summer aspect on the prairies of eastern Nebraska. Late in June it 
begins to blossom and throughout July its leaden color gives tone to the 
landscape. This perennial legume has a very extensive, woody root system 
which reaches depths of 6.5 to 7.5 feet. While some of the plants examined 
branched into large lateral roots just below the enlarged knotty crown, others 
sent vertically downward a single tap-root. These extremely tough woody 
roots have a maximum diameter of more than a centimeter. Frequently, 
large laterals run off horizontally at a depth of only 4 to 6 inches below the 
soil surface to a distance of 2.5 to 3 feet before they begin their downward 
course (plate 7, b) . Few laterals are given off until the roots reach a depth of 
from 2 to 3 feet; then the roots by dividing become much finer and branch 
quite profusely. The terminal branches are very fine, almost threadlike. 
The roots are cinnamon-brown in color; the cortex is smooth, except on older 
portions, where it is finely scaly. Five plants were examined. 

Helianthus rigidus. — ^This sunflower, common on Nebraska prairies, 
propagates by strong rhizomes, from which arise abundant fibrous roots only 
1 to 1.5 mm. in diameter. These reach a depth of about 50 inches. 

Brauneria pallida. — Only 2 plants were examined. Both had strong, 
vertically descending tap-roots which reached depths of 51 and 66 inches 
respectively. These roots were 1 or 2 cm. in diameter. They were poorly 
supplied with branches, only 3 or 4 short, weak laterals occurring on each root. 
Even the tip is unbranched. They are woody, and are chocolate-brown in color. 

Petalostemon candidus. — ^Both this plant and Brauneria are common and 
often very abundant prairie species. Like Brauneria, it has a strong, deep, 
rather poorly branched tap-root. Of the two specimens examined the smaller 



12 



THE ECOLOGICAL RELATIONS OF ROOTS. 



had a root 7 mm. in diameter and a total length of 42 inches. The larger was 
10 mm. in diameter and 68 inches deep. The laterals that do occur may- 
originate near the surface and are branched to the third or fourth order. One 
plant sent off two strong laterals at a depth of 12 inches. The deeper roots 
are very fine and quite well branched. All have a characteristic yellow color 
throughout. 

Vernonia baldwinii. — ^This weed propagates by strong rhizomes, a centi- 
meter or more in diameter, which occur about 4 inches below the soil surface. 
The roots, which are very abundant (20 or 30 arising from a single plant), 
are very different from any other roots examined. They are tough, smooth, 
yellowish-white, and unbranched throughout the first few feet of their course. 
While the stele is only about 1 mm. in diameter, the fleshy cortex is thick 
enough to give a total root diameter of from 3 to 9 mm. Many spread to a 
distance of 20 inches from the base of the plant before turning downward. 
Thus in a weedy pasture the surface soil is filled with the shiny, white, 
unbranched, rope-like roots. At about 3 feet in depth they begin to branch 
and the branches become more numerous in the deeper soils. Most of the 
finer branches and terminal rootlets, however, occur at 9 or 10 feet. Seven 
plants were examined, all of which penetrated to a depth of about 11 feet. 
The maximum depth recorded was 11 feet 6 inches. 

Kuhnia glutinosa. — This prairie plant has deeper roots than most other 
species examined. The maximum depths of 4 plants were 16 feet 8 inches, 16 
feet 6 inches, 16 feet 10 inches, and 17 feet 3 inches, respectively. A part of 
one of these is shown in plate 8, a, where only about a third of the root 
system is exposed. The root in figure 3 was reconstructed from the photo- 
graph of another root and a diagram made to scale in the field while excavating 
the plant. This diagram shows all of the larger roots, the extent of their 
lateral spread, degree of branching, etc. Although single water-content 
determinations without wilting coefficients are usually of little value, yet here 
the wetness and dryness of the various soil strata and the corresponding root 
branching were so marked as to warrant including duplicate determinations 
made on November 5, 1917, after several weeks of very dry weather. The 
clay layers at 8 and 12 feet respectively are more or less impervious, and 
collect and hold water readily. The tap-roots vary from 1.2 to 3.5 cm. in 
diameter. All the roots are nearly white in color. 

Verbena stricta. — This weed, very common in old pastures, has a strong 
tap-root from 1 to 2 cm. in diameter, which reaches depths ranging from 3.5 
to over 4 feet. Seven plants were examined. In all cases profuse branching 
occurred from the soil surface to the extreme root-tip. Because of the numer- 
ous strong laterals sent off from the tap at all levels, the latter decreases in 
size rapidly. These lateral branches lay hold of the soil to a distance of 18 
inches on all sides of the plant. Small rootlets are abundant, the whole 
forming an extremely well-developed root system. 

Grindelia squarrosa. — ^This weed has a strong tap-root abundantly supplied 
with well-developed laterals. The main root varies from 0.5 to 1 inch in 
diameter, but it gives off laterals so abundantly that this size is not long 
maintained. Maximum depths of 50, 52, 40, and 73 inches were recorded for 
the 4 plants examined. There are many very fine roots, 3 to 9 inches long, 
occupying the surface soil. The larger laterals begin to branch off in the first 
foot of soil, and run off obliquely from the main root to a distance of about 
2 feet. Like the tap, these are supplied with abundant rootlets, the whole 
forming a very extensive absorbing surface. The roots have a light yellow color. 



THE PRAIRIES. 



13 



Shantz lists Grindelia as one of the plants commonly found in the short- 
grass association of Colorado. It usually indicates disturbed conditions and 
is especially abundant during wet years. It appears that its root system 
must be greatly modified under these conditions where the soil is only rarely 
wet below a foot or two. Indeed, it seems to thrive so well and under such 
varied conditions that it is a conspicuous weed throughout many of the 
Western States. It is likely that the great plasticity of its root system makes 
this wide distribution possible. 

Glycyrrhiza lepidota. — This characteristic legume has much-branched 
rhizomes several feet long and a deep, fleshy tap-root. In the several speci- 
mens examined the tap-root varied from 0.5 to 2 inches in diameter. From 
the multicipital crown as many as 15 to 20 stems may arise. Just below the 
surface soil small laterals, a few millimeters to a centimeter in diameter, are 
often given off, in a more or less horizontal direction. These often turn down 
abruptly. At a depth of 1 to 3 feet the tap frequently breaks into many 
strong branches with a lateral spread of 2 or 3 feet from the base of the crown. 
They run off rather obliquely from the main root (plate 8, b) giving rise to 
numerous long branches. These tertiary roots are usually only 2 to 5 mm. in 
diameter and rebranch poorly, a lack of small, absorbing roots being quite 
characteristic. The deeper soils (8 to 10 feet) are thus well filled with small, 
vertically descending, poorly branched roots, 1 to 3 mm. in diameter. Like 
the older roots, these are of a light-brown color and fleshy but quite firm. 
They shrink rapidly upon exposure to the air. At 10 to 12 feet, where these 
rootlets entered the jointed clay-loess soil, they branched rather profusely. 
The roots of several plants were traced to depths of 11 or 12 feet. One of the 
larger ones was still 2 mm. in diameter at this depth and probably penetrated 
several feet deeper. The odor is very characteristic. 

Astragalus crassicarpus. — During May 1918, a large trench was dug on a 
west hillside at Belmont, about 45 feet from a wet Spartina cynosuroides zone 
in the valley. The surface foot of good loam soil gave way to about 3 feet of 
exceedingly hard joint clay, in which roots (and especially those of grasses) 
frequently followed the joints and branched largely (locally) in one plane. 
The deeper soil became wetter and also somewhat sandy, but very gummy 
and sticky. The water table was reached at about 7 feet. From this trench 
Astragalus, two species of Psoralea, and Baptisia were excavated. 

Astragalus crassicarpus forms extensive societies in the early spring. A 
group of 3 younger ground-plums was first examined ; the plants were very 
similar in size, root diameter, branching, and depth. One (shown in plate 
9, b) had a pronounced tap-root with a diameter of 1 cm. and ran almost 
vertically downward to a depth of 6 feet, giving off a number of horizontal 
branches, 1 to 2 mm. in diameter, near the surface, and extending laterally 
only a few inches before turning downward. Other smaller laterals from 0.5 
mm. to those hairlike in diameter occurred sparingly and at intervals of about 
1 cm. or less to a depth of 4 feet. Most of these branches were less than a 
centimeter in length. Below 4 feet, in the softer, wetter soil, the branches 
became more pronounced, being densely covered with root-hairs and often 
branching in two planes. However, none of these branches exceeded an inch 
in length. The tip of the tap (as well as the tips of the laterals, which reached 
depths only about half as great as the former) was no better branched than 
was the last 2 or 3 feet of the root. There is a very noticeable tendency for 
the roots to follow the crevices in the joints of the clay. 

An older plant had a strong tap-root with a diameter of 1 cm. At a depth 
of 2 cm. it gave off a strong lateral 5 mm. in diameter and at about the same 



14 



THE ECOLOGICAL RELATIONS OF ROOTS. 



level 3 other branches of equal or slightly smaller diameter (plate 9, a). They 
ran off obliquely in 4 different directions, but none to a distance greater than 
10 inches before they turned rather vertically downward; two more branches, 
each 4 mm. in diameter, were given off at depths of 6 and 8 inches respectively. 
All of these branches were very poorly supplied with laterals, as has already 
been noted for the younger plants. They taper uniformly, so that at a depth 
of 2 feet none had a diameter of more than 3 mm. The tap as well as several 
of the ultimate branches of the laterals reached a depth of 6.5 feet. The 
hairhke termini are sometimes unbranched, often for several inches, but are 
more often well-branched. Nodules 1 mm. or less in diameter occur at all 
depths, even near the root-tips. The roots are very brittle and hard to 
recover from the joint clay. They are yellowish brown in color except the 
older portions, which are dark brown. 

Psoralea tenuiflora. — This very abundant legume forms societies over large 
areas of prairie during June. Indeed, for a period these tall, coarse herbs quite 
overtop the grasses. The one shown in plate 10, a, had a tap-root 3 cm. in 
diameter, which ran vertically downward. At a depth of 27 inches it appeared 
dead, but it was traced to a depth of 52 inches, where it was still 3 mm. in 
diameter. At a depth of from 1 to 2.5 feet many laterals occur. These are 
from 7 to 12 mm. or less in diameter and frequently run off obhquely (as 
shown in the figure) for distances of 8 to 18 inches before turning, often 
abruptly, vertically downward. Several of these laterals, including some of 
the smaller ones, reached depths of 5 or 6 feet. The lateral spread is such 
that at the tips many of these roots are at a horizontal distance of from 18 
to more than 24 inches from the base of the plant. 

Little absorption occurs in the first 2 feet of soil and this entirely from lateral 
roots. The laterals are very poorly furnished with fine branches, although 
at intervals of about a centimeter short branches occur not unlike those of 
Astragalus. The root extremities, while very fine, are poorly branched. In 
color the roots are reddish-brown, except the younger portions, which are 
tan-colored. They are rather soft and more or less herbaceous, and have a 
papery bark which is readily peeled off from the older ones. 

Psoralea argophylla. — This legume, which is much less conspicuous than 
the preceding, forms estival societies over much of the prairie region. Plate 
10, B, shows the dominant, httle-branched tap-roots of several plants of this 
species. A single root will be described. This had a tap (v/ith a diameter of 
8 mm.) which tapered so rapidly that at 2 feet it was less than 1 mm. wide, 
and then pursued a vertically downward course to the water-level at a depth 
of 6 feet. No large branches were given off except at a depth of 2 feet, where 
a lateral occurred which was equal in diameter to the main root. This ran 
quite parallel with the tap and at a distance of 4 to 6 inches from it, both 
reaching approximately the same depth. 

Except for the surface foot, exceedingly fine branches, not more than an 
inch in length and rarely rebranched, occurred at intervals of about 5 mm. 
throughout the entire course of the root. At 3 inches from the tip the roots 
each divided into 2 or 3 branches, aU of which were branched to the first order 
only. The roots are dark brown to black in color. 

JBaptisia bracteata. — This showy legume, which forms vernal societies, has 
very characteristic roots, the older portions being greenish-yellow and the 
younger ones orange in color. From the base of the crown the roots spread 
out obhquely and run downward, but in such a manner that the most widely 
spreading were less than 18 to 24 inches horizontally away from the base of 
the crown at any depth (plate 9, c). The main roots are poorly branched 



THE PRAIRIES. 15 

and there are scarcely any absorbing laterals in the first 2 or 3 feet of soil. 
In the deeper soil they branch irregularly but not very repeatedly, many of 
the roots reaching the water-level at about 80 inches. It is probable that in 
drier soil they would penetrate deeper. The smaller roots near the tips (1 or 
2 feet of the extremities) often form brownish, hairlike branches which may 
run 12 to 18 inches without giving off laterals or, on the other hand, may be 
profusely supplied with small branches. Not infrequently the root-ends 
break up into two or three pieces, all of which are supplied with laterals. In 
general the extremities are covered with fine, short branches which extended 
to the water-level. 

THE SUBCLIMAX PRAIRIE. 

During the last week in March of 1918, a number of root systems 
were studied near Peru, Nebraska, at a station about 60 miles south- 
east of Lincoln. The prairies in this region are very similar in floristic 
composition to those near Lincoln, as has been shown by the studies 
of Thornber (1901) and others. Root systems of a number of species 
were studied in a prairie area covering the exposed southeast slope of a 
loess hill. A number of list quadrats which were made during the 
preceding summer revealed the dominance of Andropogon furcatus 
and A. scoparius. Indeed, the striking feature of the vegetation is the 
luxuriant growth of these bluestems. Andropogon furcatus extends to 
the very crest of the ridge. Here the stems reach heights of 5.5 feet, 
w^hile the roots penetrate the mellow loess soil to a maximum depth of 
9 feet 3 inches. This root penetration exceeds by 2 feet the maximum 
depth recorded for any plant of this species in clay-loam soil. 

As pointed out by Clements, this type of prairie, lying in a region 
of somewhat higher rainfall, is probably subclimax. The luxuriant 
growth of Andropogon upon the high ridges indicates conditions very 
favorable for chaparral growth and (as indicated later) thickets of 
Corylus, Rhus, and Symphoricarpos are very frequent in this grass- 
land. It is not uncommon also to find seedlings of elm and oak near 
the edges of these thickets. Indeed, except for fires, grazing, or other 
disturbances, much of this grassland area would probably become 
chaparral and forest. 

Other species examined near the hilltop were Brauneria pallida 
and Lygodesmia juncea. Near the foot of the slope, and where an 
abundance of clay makes the soil much harder and more compact, 
specimens of Lespedeza capitata, Amorpha canescens, and Ceanothus 
ovatus were excavated. 

Brauneria pallida. — As is frequently the case in eastern Nebraska prairies, 
this perennial herb, while never abundant, occurs in such numbers as to be a 
conspicuous component of the prairie flora. Two plants were examined. 
The smaller had a tap-root 5 mm. in diameter and reached a depth of 5.5 
feet; the larger, with a diameter of 11 nmi., reached a depth of 8 feet. The 
strong tap pursues a vertically downward course, tapering very slowly. While 
the older parts (the first 2 to 5 feet) are more or less woody and chocolate- 



16 



THE ECOLOGICAL RELATIONS OF ROOTS. 



brown to almost black in color, the younger parts are lighter in color and 
herbaceous. Short laterals (about 9 cm. in length and mostly devoid of 
branches) occur 5 to 25 mm. apart. This branching may continue to the tip, 
which is about 0.3 mm. in diameter and is poorly branched. On the larger 
specimen two laterals occurred (plate 11, a). The larger of these ran off at a 
depth of 5 feet and at an angle of about 60° to a distance of 28 inches. It was 
almost destitute of branches. The roots are tough and easily followed. The 
coal-black streaks within the cortex and stele make their identification easy. 
Although extending somewhat deeper, these specimens correspond rather 
closely with those examined in the clay-loam soil at Lincoln. 

Lygodesmia juncea. — This perennial stem-xerophyte is common in prairies 
throughout Nebraska and is often abundant on the crests of ridges or other 
dry situations. A trench about 8 feet long and 3.5 feet wide was dug on a 
slope just below a group of these plants and more than a dozen roots were 
examined. The tap-root, varying from 2 to 6 mm. in diameter, may give rise 
to several plants by means of short branches 2 to 8 inches below the surface 
(plate 11, b). 

The roots descend in an almost vertically downward course and frequently 
in parallel groups only a few inches or indeed a few milhmeters apart, to 
distances of 15 to 20 feet or more. At these depths the tips are frequently 
only 1 or 2 feet from a vertical line with the top. The roots are fleshy, very 
brittle, especially after a depth of 3 or 4 feet is reached; they are from hght 
cream to dark brown in color and exude a white latex upon injury. The side 
of the trench was dotted with drops of latex, showing the abundance of the 
cut roots. This character aids greatly lq recovering the broken ends of these 
brittle roots. Branching occurs not at all, except for tiny laterals less than 
1 mm. in diameter and an inch in length, which come off very sparingly at 
almost right angles and at intervals of 6 to 12 inches. These tiny laterals are 
poorly or not at all branched, the secondary branches being only 1 to 2 mm. 
long. The tap narrows slowly and is frequently 2 mm. in diameter at a depth 
of 16 to 18 feet. Numerous roots occurred at 18 feet in depth and one was 
traced to a maximum distance of 20 feet 7 inches. Here it was still 2 mm. in 
diameter. Because of the danger of caving of the mellow loess soil, deeper 
excavation was abandoned. The loess was of very uniform texture through- 
out and well moistened to the depth examined. 

Lespedeza capitata. — This tall, conspicuous legume is comanon throughout 
central and eastern Nebraska. It is very abundant on the lower slopes of the 
loess hill, where 6 or more plants were examined. The very characteristic 
light-yellow roots are much branched just below the surface, where numerous 
strong laterals, 5 to 7 mm. in diameter, run off in all directions, some almost 
horizontally at depths of only 3 to 6 inches. These large laterals branch very 
irregularly but profusely, giving rise to numerous finely branched roots which 
fill the surface soils to a distance of 2 or 3 feet on either side of the plant, when 
the main branches may turn abruptly almost vertically downward (plate 
11, c). In addition to these shallower roots, which compete with many of the 
grasses for the water in the surface layers of soil, numerous other branches 
pursue a more or less vertically downward course from the outset and reach 
depths of 5 or 6 feet or more. These, with the vertical extremities of the 
horizontal laterals, which may also reach similar depths, furnish a very exten- 
sive absorbing area for the plant. The deeper roots taper rapidly, but all are 
furnished with short rebranched laterals. Even the root-ends are well- 
branched, but no matting was observed, as in the case of several of the other 
legumes. A maximum depth of 7 feet 10 Laches was recorded. 



THE PRAIRIES. 



17 



Ceanothus ovatus. — This low shrub, with its many stems 1.5 to 3 feet high, 
is quite common on prairies, especially in the eastern part of Nebraska. It 
also occurs widely throughout the sandhills westward, and on the loess hill, 
where the following specimen was excavated, it is rather abundant. 

The plant examined was 13 years old. It had 14 stems arising from the 
enlarged crown. The woody tap-root was 1.5 inches in diameter. At a depth 
of 8 inches it gave off a large lateral and 2 inches below another which was 
equal in diameter to the tap (2 cm.) at this depth. While the first lateral and 
the tap pm'sued a rather vertically downward course (being, like all of the 
roots, very much curved and twisted, so that at a depth of 11 feet the tap was 
only 15 inches from a vertical line with the top), the second lateral at a depth 
of 7 feet was 50 inches from this vertical line (plate 12, a). Also, numerous 
small, repeatedly branching laterals and a few larger ones (6 mm. in diameter) 
came off in the surface foot of soil and ran in rather horizontal directions for a 
distance of 3 to 5 feet. In addition to these sm*face absorbing roots, both 
short and long laterals were given off at intervals at all depths, the whole 
root branching and rebranching freely. While some of the roots branched 
coarsely and ended abruptly, others formed a most delicate mass of absorbing 
rootlets. As a whole, the root system is well-branched, but some of the roots 
at a depth of 10 or 11 feet (and where they were only 2 or 3 mm. in diameter) 
ran 2 or 3 feet without giving off any branches. Numerous roots occurred 
at a depth of 8 feet, several reached 12 feet, and the longest one was traced to 
a depth of 14.5 feet, where it was still 1 nam. in diameter and giving off fre- 
quent threadlike laterals. The older parts of the root are woody and extremely 
hard. Deeper down they lose their toughness and become very brittle, while 
the smaller roots are again fairly tough. All parts of the root are character- 
ized by a reddish-brown color, all but the oldest being more or less streaked 
with white. They reveal a reddish color upon removal of the bark, this color 
also extending into parts of the wood. 

Amorpha canescens. — A 7-year-old specimen, which is very representative 
of others examined, was excavated near the foot of the loess hill. It gave off 
11 large woody roots, 8 to 14 mm. in diameter, from the knotty crown. These 
ran off at various angles from almost parallel with the hillside to almost 
vertically downward (plate 12, b) where the roots are somewhat grouped, 
owing to the fact that the background was only 5 feet wide. These have 
very few surface laterals, but those that do occur are well-branched. In the 
surface 2 to 4 feet of soil relatively httle absorption takes place. Andropogon 
scoparius and other grasses frequently grow vigorously between these spread- 
ing Amorpha roots and doubtless suffer little competition for water. The 
roots taper uniformly and at a depth of 3 feet are often still 7 mm. in diam- 
eter. They frequently pursue a rather tortuous coiuse and branch in a way 
more or less dichotomous. Rebranching gives rise to many small laterals 
only 1 to 4 mm. in diameter, which pursue a vertically downward course for 
many feet, giving off very few branches and tapering only slightly. Even the 
tips are usually poorly branched. The lateral spread of the roots is remark- 
able. One lateral reached a depth of 12 feet 10 inches and a horizontal dis- 
tance of 4 feet from the base of the crown. Another was also 4 feet from a 
vertical line with the crown at a depth of 12 feet 2 inches and about 5 feet from 
the end of the first lateral. Similar conditions obtained on the other sides of 
the plant, so that a very large area of soil was penetrated by the roots of a 
single plant. 

Small nodules only 1 mm. in diameter occur as deep as 10 or 12 feet. On 
the higher slope one plant was noted that showed much-branched root-tips 



18 



THE ECOLOGICAL RELATIONS OF ROOTS. 



and nodules at a depth of 16.5 feet. The roots are smooth and chocolate- 
brown in color, all being rather tough and the older parts distinctly woody. 
Here again it should be noted that the plants growing in the mellow loess 
reached depths 4 or 5 feet greater than those examined in the clay-loam soil 
at Belmont. 

PRAIRIE ROOT SYSTEMS AND THE PRAIRIE ENVIRONMENT. 

The most obvious conclusion from a consideration of these data is 
the fact that prairie species are provided with well-developed, deep- 
seated, and extensive root systems. Upon the basis of root depth 
the 33 species examined may be divided into three groups : 

1. Shallow-rooted plants are those that seldom extend below the 
first 2 feet of soil. These consist wholly of grasses, such as Kceleria 
cristata, Stipa spartea, Elymus canadensis, Distichlis spicata, Sporoh- 
olus longifolius, and Aristida oligantha. 

2. Plants with roots extending well below the second foot of soil but 
seldom deeper than 5 feet may be grouped as intermediate in root 
depth. Here belong Andropogon scoparius, A. nutans, Bouteloua gra- 
cilis, Bulbilis dactyloides, Verbena stricta, Helianthus rigidus, Solidago 
rigida, and Petalostemon candidus. Here also may be placed Grindelia 
squarrosa, a plant which sometimes extends beyond this depth. 

3. Of the plants studied, 55 per cent have roots which extend beyond 
a depth of 5 feet ; indeed, most of them to depths of from 7 to 9 feet and 
a few to a maximum depth of from 13 to 20 feet or more. These may 
be classed as deep-rooted species. Here belong Panicum virgatum, 
Andropogon furcatus, Agropyrum repens, Solidago canadensis, Liatris 
punctata, Silphium laciniatum, Amorpha canescens. Astragalus crassi- 
carpus, Psoralea tenuiflora, P. argophylla, Lygodesmia juncea, Ceanothus 
ovatus, Baptisia hracteata, Lespedeza capitata, Glycyrrhiza lepidota, 
Brauneria pallida, Vernonia haldwinii, and Kuhnia glutinosa. 

To understand the causes for such remarkable root development 
it will be necessary for us to study the prairie environment. The 
prairies of eastern Nebraska receive more moisture than most of the 
great grassland area. The mean annual precipitation for Lincoln, 
together with its seasonal distribution (which is of greater ecological 
significance), is shown in table 1. 

It may be seen that most of the precipitation falls during the growing 
season and less than one-tenth of it during the three winter months. 
About half of the rainfall of May, June, and July is from rains of an 
inch or more in 24 hours. Such a seasonal distribution of moisture is 
very favorable for the growth of grasses. Not infrequently, however, 
storms occur with a rainfall exceeding 2 inches and occasionally 4 or 5 
inches in a period of 24 hours. Such storms invariably result in a high 
run-off and they account largely for the observed deficiencies of mois- 
ture for crops in seasons where the recorded rainfall would indicate an 
abundant supply. Drought periods of 30 or more consecutive days 
between March 1 and September 30, in which precipitation to the 



THE PRAIRIES. 



19 



amount of 0.25 inch does not occur, are not infrequent. Indeed, 16 or 
17 such periods have occurred at Lincoln during the past 22 years. 
The average annual snowfall is about 24 inches. ^^As a rule snow 
covers the ground but a few days at a time after each snow storm, and 
the ground is covered with snow less than half of the time even during 
the months of the heaviest snowfall" (Loveland, 1912). Much of 
the snow is swept by high winds into depressions, and thus contributes 
often but httle to the supply of soil moisture of the land upon which 
it falls. Hence it may be seen that precipitation is only a general 
indicator of conditions for plant growth. Obviously its influence upon 
the distribution and seasonal activities of plants is exerted through its 
power to replenish soil moisture. 

Table. 1. — Mean monthly and annual precipitation at Lincoln, Nebraska, in inches. 



Months. 


Precipi- 
tation. 


Months. 


Precipi- 
tation. 


Feb 


0.67 
0.96 
1.26 
2.51 
4.39 
4.43 


July 

Aug 

Sept 

Oct 


4.01 
3.72 
2.91 
1.94 
0.94 
0.85 

28.59 


May 


Dec 

Annual. . . 



The fertile, dark-colored prairie soil of the region is of the type com- 
monly called loess, much of which, however, is confounded with glacial 
drift. ^'The loess covers the hills and valleys alike to a depth of from 
20 to 100 feet, being much thicker than this in places and much thinner 
in others. Throughout the first 100 miles westward from the Missouri 
it is underlain by Kansan till " (Alway, 1916) . ''The uniformity in the 
physical properties, recognized as characterizing the material of the 
loess, should tend to produce, under uniform climatic conditions, soils 
uniform in chemical properties." The water-holding capacity of the 
surface foot of soil is about 60 to 70 per cent of its dry weight, while the 
moisture equivalent and wilting coefficient are 25 per cent and 13.5 
per cent respectively.^ 

Studies of the water-content of upland prairie soils have been 
carried on for more than two seasons. Table 2 gives the results of 
water-content determinations during the growing season of 1916. 
The minus sign indicates water non-available for plant growth. 

A glance at these results shows that at four different periods no 
water was available for growth at a depth of 4 inches, while during 
late August the same condition obtained for the 4 to 12 inch layer. 
Unfortunately, deeper soil samples were not obtained, but during the 
following seasons samples were taken at irregular intervals to a depth 
of 5 feet. These data are shown in table 3. 

^ The writer is indebted to Dr. L. J. Briggs, of the Bureau of Plant Industry of the U. S. 
Department of Agriculture, for determinations of moisture equivalents and wilting coefficients. 



20 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Table 2. 

In tables showing water-content it will be understood that the figure columns represent 

percentages. 





Depth of samples. 




Depth of samples. 


Date. 


to 4 inches. 


4 to 12 inches. 


Date. 


to 4 inches. 


4 to 12 inches. 




Wilting 


Wilting 




Wilting 


Wilting 




coefficient 12. 


coefficient 12.7. 




coefficient 12. 


coefficient 12.7. 


May 21 


19.5 


17.3 


July 31.... 


- 0.8 


0.1 


June 5 


- 2.5 


4.8 


Aug. 7 


9.5 


8.8 


June 10. . . . 


- 3.5 


3.8 


Aug. 15 ... . 


20:0 


13.8 


Jime 17 


3.5 


9.1 


Aug. 22 


- 2.2 


-2.7 


June 26 


16.5 


9.5 


Aug. 28 


- 1.7 


3.8 


July 1 . . . . 


1.3 


5.5 


Sept. 4.... 


13.1 


8.8 


July 8.... 


- 2.4 


2.0 


Sept. 12 


13.5 


5.3 


July 17.... 


9.5 


2.8 


Sept. 19 


2.0 


1.8 


July 24 


- 1.2 


0.3 









Table 3. — Available water-content of the prairie soil during 1917. 
The minus sign indicates water non-available for plant grov.'th. 



Date. 


Depth of 
sample, 
to 6 
inches. 
Wilting 
coefficient, 
13.4. 


Depth of 
sample, 
6 to 12 
inches. 
Wilting 
coefficient, 
13.4. 


Depth of 
sample, 

1 to 2 

feet. 
Wilting 
coefficient, 

15.4. 


Depth of 
sample, 
2 to 3 
feet. 
Wilting 
coefficient, 
14.5. 


Depth of 
sample, 
3 to 4 
feet. 
Wilting 
coefficient, 
16.1. 


Depth of 
sample, 
4 to 5 
feet. 
Wilting 
coefficient, 
16.1. 


Apr. 3 

May 6 


4.3 
-2.9 
4.9 
-1.9 
-1.7 
-3.1 
-5.3 
1.6 
7.0 
2.1 


4.3 
6.7 
9.8 
2.0 
0.7 
-1.7 
-2.3 
-1.3 
0.5 
1.7 


-2.6 
2.2 
8.1 


-1.6 
-1.4 
8.4 


-2.9 


-0.4 


July 9 


6.1 


4.3 


July 16 


2.1 


1.7 






July 23 






July 30 


-4.0 








Aug. 6 








Aug. 13 










Aug. 28 

July 24 (1916) 


-2.1 
-0.9 
-0.9 
1.4 


-1.3 
2.3 
-0.7 
-3.9 


-4.5 


-1.9 


Jan. 5 (1918) 

Mar. 26 (1918) 


3.3 
7.2 


1.9 
7.4 


0.6 
-4.2 


2,1 
-2.8 



On May 6 and again throughout the whole of July, no water was 
available in the first 6 inches of soil. During late July and the first 
week of August soil moisture was depleted to a point below the wilting 
coefficient for a depth of 12 inches. On April 3 and again on August 28 
no water was available at a depth of 1 to 5 feet. These results are 
rather surprising, but an examination of the rainfall record shows that 
during 1916 the precipitation was 5.5 inches below normal, while during 
1917 it fell to 6.5 inches below. The dryness of the soil during 1916-17 
was shown by an abnormal amount of winter-kilHng of trees and 
shrubs. During July the rainfall was only 0.56 inch instead of the 
normal 4 inches. The determinations on July 24 (1916) at 2 and 3 
feet respectively show that the soil was very dry. It is interesting to 
note in this connection that similar results were obtained in the 



THE PRAIRIES. 



21 



prairies of southeastern Washington, where during 1914 the water- 
content to a depth of 4 feet was reduced to the non-available point 
(Weaver, 1915: 233-235). 

The available water-content in the loess soils supporting the sub- 
climax prairie vegetation was not only higher but much more con- 
stant. These data are shown in tables 4 and 5. Here also are included 
determinations from an adjacent thicket of Corylus americana. A 
comparison of these results is made on page 30. 



Table 4. — Available water-content in prairie and shrub communities near Peru, Nebraska, 
during April to September 1917 and May 1918} 

The minus sign indicates water non-available for plant growth. 



Depth, to 6 inches. 
Wilting coefficients: 
Prairie 11.7; scrub 13.8. 


Depth, 6 to 12 inches. 
Wilting coefficients: 
Prairie 10.9; scrub 12.3. 


Depth, 1 to 2 feet. 
Wilting coefficients: 
Prairie 10.4; scrub 12.5. 


Depth, 2 to 3 feet. 
Wilting coefficients: 
Prairie 9.9; scrub 11.3. 


Date. 


Prairie. 


Scrub. 


Date. 


Prairie. 


Scrub. 


Date. 


Prairie. 


Scrub. 


Date. 


Prairie. 


Scrub. 


1917 






1917 






1917 






1917 






Apr. 5 
May 15 
June 30 


13.4 
13.0 


21.4 
14.7 


Apr. 5 
May 15 
June 30 


12.2 
12.0 


18.0 
14.1 


Apr. 5 
May 15 
June 30 


8.5 
10.5 


12.1 
11.7 


Apr. 5 
May 15 
June 30 


5.0 
8.1 


8.0 


13.9 


20.8 


11.8 


17.1 


12.3 


13.1 


12.6 


12.9 


July 9 
July 14 
July 24 
July 28 
Aug. 6 
Aug. 11 
Aug. 18 
Aug. 25 
Sept. 8 
Sept. 22 
1918 


6.5 


3.9 


July 9 
July 14 
July 24 
July 28 
Aug. 6 
Aug. 11 
Aug. 18 
Aug. 25 
Sept. 8 
Sept. 22 
1918 


7.3 


7.4 


July 9 
July 14 
July 24 
July 28 
Aug. 6 
Aug. 11 
Aug. 18 
Aug. 25 
Sept. 8 
Sept. 22 
1918 






July 9 
July 14 
July 24 
July 28 
Aug. 6 
Aug. 11 
Aug. 18 
Aug. 25 
Sept. 8 
Sept. 22 
1918 






9.5 


11.5 


7.4 


5.2 










0.1 
-1.4 
4.0 
8.4 


4.5 
-1.7 

6.6 
12.6 


2.5 
0.4 
-0.5 
1.5 


3.4 
-0.4 
2.6 
5.7 


3.1 
2.1 
-1.0 


3.0 
1.2 
0.7 


5.1 
4.5 
1.9 


5.5 
4.4 
2.4 


2.3 


7.6 


1.9 


1.5 










1.3 
6.2 
-1.2 


6.4 
10.2 
-1.6 


0.2 
-0.8 
-1.7 


1.8 
2.4 
0.5 


0.6 
0.8 
-1.0 


-0.1 
-0.4 
-0.7 


0.9 
1.6 
0.1 


1.2 
-1.0 
0.5 


May 16 


13.8 


16.3 


May 16 


15.0 


18.3 


May 16 


12.5 


16.5 


May 16 


12.7 


16.5 



The writer is indebted to Prof. F. C. Jean for the major portion of the data in these tables. 



The greater water-content of these mellow loess soils compared with 
those of the prairie near Lincoln gives us a clue to the much more 
luxuriant growth of certain prairie and shrub species. There is also a 
great difference in depth of root-penetration, plants like Andropogon 
furcatus and Brauneria pallida being much deeper-rooted in loess soil. 

However, vegetation is not only an expression of present conditions, 
but also to a greater extent a record of conditions that have obtained 
during a period of years, and the record is not likely to be altered 
greatly in a year or two in which conditions may depart from the 
normal. The preceding pages show that many prairie plants absorb 
moisture well beyond a depth of 5 feet, while soil-moisture extends 
many feet beyond the greatest root depth. In excavating root systems, 
during September to December 1917, the soil below 5 feet was found 
almost invariably to be quite moist. Some of the root systems were 
excavated on bench-lands in the Salt Creek basin, where the alluvial 
soils are somewhat different from those described. Such differences 
were noted in discussing these species. 



22 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Just as the possible growth of the aerial parts of plants is affected 
by the extent of the development of the root system, conversely the 
environmental conditions to which the aerial parts are subjected, 
especially as concerns their water relations, must reflect themselves 
in the root development. Therefore, it will be instructive to consider 
briefly the above-ground environment. 



Table 5. — Total water-content of soil during 1917. 



Table 6. 



Date. 


3 to 4 feet. 


4 to 5 feet. 


Prairie. 


Scrub. 


Prairie. 


Scrub. 


1917 








p. ct. 


Apr. 5 . 


15.9 


16.6 


13.0 


16.1 


June 30. 


20.5 


24.7 


21.4 


24.9 


Aug. 6 , 


15.5 


15.4 


16.9 


16.7 


Sept. 22. 


12.2 


12.1 


11.9 


13.0 


1918 










May 16. 


13.1 


15.0 


15.4 


15.1 



Month. 


Temp. 


Month. 


Temp. 


Jan 

Feb.... 

Mar 

Apr .... 
May . . . 
June . . . 
July.... 


op 

22.6 
24.3 
37.4 
51.5 
61.7 
71.2 
76.4 


Aug 

Sept.... 

Oct 

Nov. . . . 
Dec... 


°F. 
75.0 
66.6 
54.6 
40.4 
28.5 


Aver. . 


50.8 



The mean monthly temperature at Lincoln, Nebraska, for a period of 
more than 30 years is shown in table 6. 

The season without kilhng frosts usually extends from the first day 
of May into the first week of October, but frosts have occurred as late 
as the last week of May and as early as the second week of September. 
Figure 4 shows the average daily temperatures obtained during the 
growing season of 1916, as recorded by a hygrothermograph placed 
in an appropriate shelter in the prairie at a height of 17 cm. The 
weekly means were obtained by drawing a horizontal hne through the 
weekly record sheet in such a manner that the total area above this 
hne included by the graph was equal to the total area below the Hne. 




M 


JUNE 


JULY 


AU&. 


SEP. 


























u 








i 




y 




I 


















r 













Fig. 4. — Graphs showing the average daily- 
temperature (heavy line) and humid- 
ity (light line) in the prairie during 
1916. 



Fig. 5. — Graphs showing the aver- 
age daily evaporation rates in 
the prairie during 1916 (heavy 
line) and 1917 (Ught line). 



The areas were determined by the aid of a planimeter. In this inter- 
pretation both temperature (or humidity) and the time factor are 
taken into consideration. This figure also gives a record of the mean 



THE PRAIRIES. 



23 



weekly humidity of the air during this same period. Not infrequently 
during the long summer afternoons the humidity falls to 20 per cent 
or less, while the temperature may reach 95° to 100° F. 

During the gromng season the prevailing winds are from the south 
or southeast. A standard anemometer placed in the prairie on a gentle 
southwest slope and at a height of 0.5 meter recorded 4,905 miles of 
wind from July 13 to September 19, 1916, an average of 72 miles per 
day. A similar instrument placed at the same height above the soil 
surface, but on a hill top in the same prairie area, showed an average 
daily wind movement of 122 miles per day. 

The factors of high temperature, low humidity, and wind movement 
combine to increase the evaporating power of the air. Livingston's 
porous-cup atmometers, fitted with non-absorbing mountings, were 
operated in the usual manner in duplicate and at a height of about 17 
cm. above the soil surface during 1916 and 1917. The combined 
evaporation records from two prairie stations, reduced to the readings 
of the standard cup, are shown in figure 5. During the last week in 
July 1916, when the mean daily temperature was 81.4° F. and the 
mean relative humidity 57.4 per cent, the daily evaporation was 38 
c.c. During this period no soil moisture was available in the first foot 
of soil. The average daily evaporation throughout this whole summer 
was 21.6 c.c. Atmometers exposed at a height of 0.5 meter during the 
last 10 days in July gave losses over 50 per cent greater than similar 
instruments placed at the usual height of 17 to 23 cm. In the prairies 
near Peru, Nebraska, the average daily evaporation from May 15 to 
September 22, 1917, was 20.9 per cent. 

Summarizing briefly, we find that prairie plants of this region grow 
under semi-arid climatic conditions in which the supply of water is the 
chief limiting factor of plant growth. During certain portions of the 
gromng season extremely xerophytic conditions are brought to bear 
upon the vegetation. It has been shown that the water-content of the 
soil is reduced to the non-available point to a depth of 4 or 5 feet at 
least during certain years and often at times when the evaporating 
power of the air is very high (38 c.c. daily). 

In response to these environmental conditions many species have 
developed surprisingly extensive root systems. In fact, all of the 
dicotyledons examined, as well as many of the grasses, extend their 
root systems to depths of from 3 to 7 feet or more. For just as the 
evaporating power of the air and the nature of the transpiring organs 
determine the water-loss of plants, Hkewise the soil water and the nature 
of the root systems determine the supply. These findings of great root 
depths, which are correlated with deep soil moisture, bear out Can- 
non's suggestion of the probabihty that the longest or most deeply 
penetrating roots are found, not in deserts, but where there is consider- 
able rainfall, and where the penetration of rain is considerable and the 
water-table relatively deep. 



24 



THE ECOLOGICAL EELATIONS OF ROOTS. 



The roots of prairie plants are grouped into more or less definite 
absorbing layers, many of the deeper-rooted species having few or 
no absorbing roots in the first few feet of soil. This layering of the 
roots reduces competition and permits the growth of a larger number 
of species. Any of the shallow-rooted grasses extract water from the 
soil layers quite above those from which Andropogon furcatus or 
SoUdago rigida draw their supply, while still deeper soil strata furnish 
moisture for such deep-seated species as Panicum virgatum, Silphium 
laciniatum, and Kuhnia glutinosa. It should be pointed out here that 
prairie plants very seldom show the widely spreading and superficial 
laterals so characteristic of many plains species. The cause seems to 
lie in the greater water-content of the deeper soil in the prairie. 

Root variations of prairie plants may be due to a number of factors, 
among which the water-content of the soil and its penetrability prob- 
ably stand first in importance, at least in non-alkaUne soils. Although 
the problem of the causes of root variation is one which must be solved 
under control conditions where one factor can be varied at a time, 
considerable evidence has been found in the field that both soil mois- 
ture and soil texture profoundly influence root distribution. 

The effect of compact soils upon root penetration is shown not only 
by the tortuous courses pursued by roots and the distortions of the 
roots themselves, but also by their modified branching. Several 
species, including grasses, were found to penetrate from 2 to 3 feet 
deeper in deep loam soil than in the same type of soil when it was 
underlaid with a hard clay subsoil. In soils with a subsoil of alternating 
layers of sand and clay a striking distribution of lateral roots was 
observed. These often occurred abundantly near the bottom of the 
sandy layers and in the clay strata where the latter acting as a rather 
impervious layer had retained much soil moisture. 

It is interesting to note that species of the same genus may have 
an entirely different type of root system. Liatris punctata has a 
deep tap-root, while the multitudinous roots arising from the corm of 
L. scariosa are fibrous. Likewise, the roots of Andropogon scoparius 
are as different from those of A. furcatus as are the roots of Kceleria 
cristata from those of Stipa spartea. Indeed, the roots studied are 
remarkable for their individuality. The roots of each species, because 
of peculiarities of form, branching habit, position in the soil, texture, 
color, odor, or taste, can be easily distinguished, and these distinguish- 
ing characters have often proved useful in ecological work. In some 
cases it has been possible to reconstruct successional changes from the 
Tecord left in the soil by partially preserved roots and rhizomes. 



THE CHAPARRAL COMMUNITY. 



25 



II. THE CHAPARRAL COMMUNITY. 

Between the great Ohio-Missouri deciduous forest complex on the 
east and the prairies to the westward, there extends a chaparra 
community. Indeed, this is more or less continuous from Canadl 
to Texas. In this shrub community, tongues of which extend far into 
the subchmax prairie, species of Symphoricarpos, Rhus, Corylus, and 
Rosa play the role of dominants. Indeed, all but the latter form 
thickets of greater or less extent in the moister places throughout much 
of the prairie community or occur as more or less isolated clumps or 
individuals held in check only by the severe root competition of the 
prairie species (plate 16, b). They rapidly spring into dominance, 
suppress the grasses, and form centers for further outward invasion, 
when soil-moisture conditions are even shghtly increased above the 
normal. This may be due in the first instance to the loosening of the 
soil by burrowing animals, the building of a fence, or other disturbance 
of the sod. 

In order to understand more clearly the nature of the competition 
between scrub and grassland, as well as to determine more exactly 
environmental conditions indicated by these phyads, a number of root 
systems were examined along the loess hills of the Missouri River near 
Peru, Nebraska. 

Symphorlcarpos vulgaris. — ^This species, together with its western ecological 
equivalent, S. ocddentalis, is a very conspicuous and important shrub of the 
chaparral community. Growing in clumps to a height of only 2 to 4 feet, its 
shade is frequently so dense as to exclude even the very tolerant Poa pra- 
tensis. In a well-established shrub area the latter almost invariably occurs 
between the individual clumps. 

A long trench was dug at the edge of such a thicket and the roots of numer- 
ous plants were examined. The larger roots arise mostly from near the base 
of the erect shoots, but numerous smaller ones occur, especially all along the 
underground stems (plate 13, a). The maximum depth to which the roots 
penetrate in the loess soil is only 65 inches. Although the trench was simk to 
a depth of about 8 feet and a part of the soil underlying the thicket at this 
depth was removed, no roots of Symphoricarpos were found below the 65-inch 
level. Indeed, except for a few plants of Rhus, competition for Ught above 
ground was so intense that no other plants were present and the deeper soil 
was free from roots of any kind. But lack of linear extent is amply recom- 
pensed by a wonderfully well-developed absorbing surface. Perhaps the roots 
of no other plant examined, with the exception of certain surface-feeding 
grasses, occupy the soil more thoroughly than does the deHcate network of the 
root branches of this shrub. 

The larger roots vary from 3 to 7 mm. in diameter. While they may come 
off vertically, usually they pursue an oblique direction for some distance 
(1 to 3 feet) before turning downward (plate 13, a). These reddish-brown, 
tough, woody roots taper imiformly, frequently giving off large branches and a 
network of finer ones, beginning just beneath the surface of the soil. Indeed, 
the roots are profusely and minutely branched and rebranched throughout, 
the ultimate branches being almost microscopic in size. The laterals vary 



26 



THE ECOLOGICAL RELATIONS OF ROOTS. 



in length from those only an inch long to others extending a distance of 2 or 
more feet, sometimes almost at right angles. The last foot or 18 inches of the 
root is usually threadlike but extremely well branched. Above the level of 
65 inches the soil is well filled by great numbefs of these fine root-endings. 

Rhus glabra. — trench 10 feet long was dug just within a thicket of sumac 
which, according to ring counts, had been in possession of the area for at least 
12 years. Practically all herbaceous species had disappeared except Poa 
pratensis. A brief examination showed the soil to be filled with a network of 
rhizomes from which numerous roots ascended more or less vertically and 
ended at the surface of the soil in a brush-like mass of branches. Further 
study revealed the really wonderful complexity of these underground parts 
as well as the great absorbing area of this xero-mesophytic shrub (plate 
14, A, b). Roots were examined in three different situations. Although they 
differ in details, the following is a tjrpical example : 

A tap-root 3 cm. in diameter and 12 or 13 years old pursued a downward 
course at an angle of about 40° from the vertical to a depth of 22 inches. Here 
it turned off at an angle of 30° from the horizontal and ran a distance of 2 
feet, at which length (48 inches) it was only 1.5 cm. in diameter and 37 inches 
deep. Here it forked into two equal branches, one (a) running almost hori- 
zontally to a distance of 68 inches and at a depth of 3 feet, while the other (b) 
turned off at an angle of 40° from the first and, after running rather obliquely 
downward for a distance of 15 inches, gave off two horizontal laterals 3 and 6 
mm. in diameter respectively, while the main root turned vertically upward. 
It ran in this direction for a distance of 8 inches, when it divided into 3 equal 
branches. Two of these were each 6 mm. in diameter and tapered gradually 
till they reached the surface, 26 inches above, and at a horizontal distance of 
26 inches beyond the vertical root from which they branched. They ended 
about 2 feet apart in an extremely well-branched and rebranched network 
of fine rootlets. These root termini were 70 inches in a horizontal line from 
the beginning of the tap. 

The third branch from the upright portion of the root turned off horizon- 
tally and ran a distance of 8 inches, when it again turned at right angles and 
ran horizontally in another direction (away from the main tap) 16 inches, 
finally turning downward at right angles and descending to a depth of 80 
inches. The tip, which ended in a much-branched network of almost hair- 
like terminals, was located at a horizontal distance of 68 inches from the 
beginning of the tap. The two 3 and 6 mm. laterals of root (6) ran off hori- 
zontally, but in a direction opposite from the others, finally ascending to the 
surface several feet from the other roots. Branch (a), which was partly cut 
off in digging the trench, also sent off mmaerous roots to the surface as well as 
some into the deeper soil. The surface soil is filled with rootlets with such a 
network of absorbing terminals that competition with the grasses for soil 
moisture must be very severe. Nor is the main absorbing area confined to the 
surface. Well-branched laterals, both large and small, arise at all depths, and 
even the deeper soil is rather thoroughly occupied. The well-branched root 
system, together with the large area it occupies, must afford abundant mois- 
ture, even in fairly dry soil. 

While all of the main roots examined in two separate thickets showed the 
obhque to horizontal course with a depth never exceeding 80 inches, one root 
of a Rhus plant growing in the Symphoricarpos thicket was 0.5 inch in diam- 
eter and pursued a vertically downward course to a depth of over 90 inches. 
The roots are characterized by a thick cortex which exudes a white latex when 
injured, and upon removal reveals the small, woody, glistening white stele. 
The younger roots vary in color from brown to nearly black. 



THE CHAPARRAL COMMUNITY. 



27 



Corylus americana. — This shrub does not get so far into the prairies from 
the bordering eastern forests where it forms a layer as do the preceding, and 
in this respect shows its less xerophytic habit. But in many situations, from 
Minnesota to Kansas, thickets of Corylus come into direct competition with 
prairie vegetation. 

Corylus spreads by means of large woody underground parts, as shown in 
plate 15, c. These run at a distance of only 4 to 6 inches under the surface 
and give rise at intervals to numerous erect shoots from 2 to 7 feet high and 
also to multitudinous roots, some of which are more than 1.5 cm. in diameter. 
Some of the smaller roots run vertically toward the surface and branch pro- 
fusely into very fine ultimate branchlets, for Corylus, Uke the preceding 
shrubs, is a strong competitor of the grasses for moisture in the surface soil. 
Indeed, the surface foot or two of soil is well filled with absorbing rootlets, the 
longer roots being only fairly well supplied with laterals to a depth of 10 or 11 
feet. These larger roots, as shown in plate 15, a, either pursue a rather verti- 
cally downward course, or extend obhquely for a distance of 1 or 2 feet or 
more and then turn downward. Roots were traced to depths of 10, 10.5, and 
11.5 feet respectively. They are dark brown in color, very woody, with a very 
thin cortex, and frequently branch rather dichotomously, although the 
branches are seldom equal in size. The branches are often very coarse and 
sometimes run in parallel groups several inches without giving rise to absorb- 
ing rootlets. The main root-ends are not very well supphed with fine branches. 
Thus it may be seen that while Corylus penetrates to greater depths than 
Rhv^ or SymphoricarpoSy its absorbing system is hardly developed to such a 
high degree. 

Vitis vulpina. — At a depth of over 10 feet in the Rhus thicket a root of 
Vitis was encountered. It was 18 mm. in diameter and ran horizontally 
across the trench. It gave off numerous branches, some of which reached 
depths of 12 feet. Its branching is not unhke that of the larger roots of the 
hazel. In the Corylus thicket, roots of grape were traced to depths of 13 feet 
3 inches and 13 feet 6 inches respectively. The diameter of roots near the 
surface was about 1 cm. ; at 10 feet it was 2 and 3 mm. respectively. Another 
underground part ran horizontally at a depth of 2 feet for a distance of more 
than 6 to 8 feet. Such an enormous absorbing area as is possessed by Vitis 
gives us the clue to its ability to have a leaf area not unlike that of many trees 
and yet be able to five often in apparently rather dry situations. 

Rosa arkansana. — This shrub is widely distributed throughout the scrub 
and prairie areas. Although usuallj^ held in check in the dense grassland sod, 
except where local disturbances have favored its development, in the sub- 
climax grassland it frequently forms dense brush over the less-exposed slopes. 
In the sandhills westward it again assumes an important role, areas several 
acres in extent frequently being controlled by this species. 

As shown in plate 16, a, this plant propagates by rhizomes. This parent 
plant had given rise to 5 distinct clumps of stems, the youngest of which was 
5 years old and more than 5 feet from the oldest. Fragments of other hori- 
zontal parts, which had not yet developed shoots, may be seen. It may also 
be noticed that the plant next to the parent is the only one that has developed 
a tap-root of its own. Indeed, the other roots arising from the horizontal 
portion ran obhquely upward, rather horizontally or, at most, only very 
obliquely downward, and none reached any considerable depth when com- 
pared with the taps from the older plants. The tap-root from the second 
plant reached a depth of 15 feet 2 inches; the main tap pursued a nearly ver- 
tically downward course to a depth of 21 feet 2 inches. 



28 



THE ECOLOGICAL RELATIONS OF ROOTS. 



It should be noted here that these roses grew about midway up a southeast 
slope. Here the loess soil was intermediate between that described for 
Lygodesmia near the crest and for Amorpha near the foot of the hill. Beyond 
a depth of 10 feet the soil was quite compact. 

Plate 15, B, illustrates well the paucity of large branches. The lateral 
spread of any branch measured horizontally from the base of the crown did 
not exceed 4 feet. Although many fine branchlets occurred along the course 
of these main roots and extended off laterally for distances of 6 to 18 inches, 
still other portions were quite free from branches. The breaking up of the 
larger roots near their extremities into numerous long, slender, often more or 
less parallel rootlets is well shown on the root in the figure, which ends at a 
depth of about 6 feet. 

The older woody roots can easily be identified by scraping off the outer 
black part of the thin cortex, which then reveals a bright red color, while 
similar treatment of the younger roots shows their orange color. 

SHRUB ROOT SYSTEMS AND THE SHRUB ENVIRONMENT. 

From the preceding descriptions it may be seen that the members 
of the chaparral community of the tension zone are all supplied with 
splendid absorbing systems, which are somewhat variable as to depth, 
but all of which are deep-seated. In addition, all have excellent 
methods of vegetative propagation. 

The role played by these shrubs in modifying prairie conditions to 
such an extent that the habitat becomes rather favorable to the growth 
of the more xerophytic trees, such as bur oak, should not be overlooked. 
All are well adapted, either by means of above-ground or under- 
ground stems or root offshoots, to slowly but successfully invade the 
prairie sod. For example, the whole loess hill, where these shrubs were 
excavated, would probably be covered by a chaparral complex except 
for repeated fires which are more detrimental to shrubs than to grasses 
and most other prairie species. The proof of this statement lies not 
only in the presence even over much of the more exposed hillside of a 
potential scrub mictium, but also in the fact that Rhus, Symphori- 
carpos, and other shrubs domiiiate over other portions of the same 
ridge where soil conditions are identical but where fires have not been 
permitted to run. 

Synphoricarpos extends its area by migrating by above-ground stems 
as well as by those below the surface. This mechanism of invasion 
is well illustrated in plate 13, b. While the above-ground stems furnish 
the more rapid method of migration, they frequently fail to become 
rooted in the prairie sod. They then become greatly attenuated and 
soon die. The under-ground stems are more certain of establishment. 

Once established, this shrub rapidly reacts upon the habitat by 
increasing the water-content both of air and soil, and also by modifying 
the texture and composition of the latter. The presence of the shrubs 
decreases wind movement and their shade reduces not only the light 
but the temperature as well. This reacts favorably upon the humidity 



THE CHAPARRAL COMMUNITY. 



29 



and also decreases the evaporation from the soil surface. The reduc- 
tion of the evaporating power of the air within a Symphoricarpos area 
at a height of 17 cm. as compared with that 2 meters beyond in the 
prairie is well illustrated by the following data, giving the average 
daily evaporation in a clump of Symphoricarpos and in the adjacent 
grassland, both near the crest of a loess hill : 



Date. 


Prairie. 


Scrub. 




c. c 


c. c. 


July 1 to 7, 1918 


33.1 


25.6 


July 29 to Aug. 5, 1918.. 


46.0 


33.8 


Aug. 26 to Sept. 4, 1918. 


18.0 


14.6 



Not only is the evaporating power of the air lowered by the presence 
of the scrub, but among its stems drifts of snow find lodgement and 
upon melting add considerably to the moisture content of the soil. 
The run-off is greatly reduced as a result of the rich mulch of fallen 
leaves and large quantities of wind-transported plant debris lodged 
among its stems. Only a few years are required, owing to the more 
favorable conditions for the formation of humus under the shrubs, to 
fill the former prairie soil with a rich humus mulch not unlike that of 
the woodland. Thus by cumulative favorable reactions the scrub 
is frequently able to extend its area of dominance and also to prepare 
a fine nursery for the seedhngs of trees, thus initiating a new stage in 
the normal succession. Repeated determinations of water-content to 
a depth of 3 feet show that the thicket soil is moister than that of the 
prairie. 

The reactions of Rhus upon the habitat are similar to those de- 
scribed for Symphoricarpos. Being a taller shrub, its effect upon the 
bordering grassland is more pronounced. It invades the prairie by 
means of rhizomes. These have been traced at a depth of only 4 to 
8 inches to a maximum distance of over 20 feet from the edge of a 
thicket, their course being marked by the presence of erect shoots. 

The evaporating power of the air is greatly reduced under Rhus as 
compared with that in the prairie^ as is shown by the following experi- 
ment: From May until September, 1916, non-absorbing atmometers 
placed just 6 meters within the sumac thicket gave evaporation losses 
averaging about 10 c.c. per day as compared with average daily losses 
of 22 c.c. from similar instruments 24 meters outside the thicket in the 
prairie. Likewise, the water-content of the soil at the scrub station 
as compared with that of the grassland was found to average about 
6 per cent higher throughout the season to a depth of 10 inches. The 
deeper layers of soil to 5 feet were invariably much moister in the 
sumac community. 



30 



THE ECOLOGICAL RELATIONS OF ROOTS. 



The profound effect of a growth of hazel upon modifying prairie con- 
ditions is well illustrated by a series of evaporation and soil-moisture 
readings obtained on a flat hilltop in the edge of a hazel thicket and a 
few meters beyond in the prairie. The evaporating power of the air 
measured during the growing season of 1916 and 1917 was about 50 
per cent less in the Corylus thicket. On the other hand, the water- 
content of the soil, as in the case of the other shrub communities, was 
found to be somewhat higher. 

During 1917 another series of evaporation readings and soil-moisture 
determinations was made at Peru in the loess soil. The atmometers 
gave an average daily loss of 20.9 c.c. in a prairie on a southeast slope, 
while in a Corylus thicket on a northwest slope the loss was only 14.4 
c.c. daily. This ratio was maintained almost throughout the summer 
and at no time were the losses in the shrub a^s great as those in the 
prairie. That the water-content of the soil in the shrub was usually 
much higher than that in the prairie may readily be seen by an exami- 
nation of tables 4 and 5 on pages 21 and 22. 



THE PRAIRIES OF THE PACIFIC NORTHWEST 



31 



III. THE PRAIRIES OF THE PACIFIC NORTHWEST. 

The prairies of southeastern Washington and adjacent Idaho, 
where the following studies were carried on, represent an extreme 
westward extension of the great grassland formation lying east of the 
Rocky Mountains. Agropyrum spicatum, Festuca ovina ingrataj 
Koeleria cristata, and Poa sandhergii are dominants. Stipa is entirely 
absent, as are also the late-blooming grasses, such as Andropogon, 
Bouteloua, and others of the eastern prairie. The absence of these 
late-maturing grasses may be accounted for by the peculiar distribu- 
tion of the precipitation. Only about one-fourth of the annual 21 
inches of moisture falls during the growing season. Except for the 
extremely retentive silt-loam soil, the region would be almost a desert. 
This is an extremely important fact to keep in mind while consider- 
ing root systems. The important role played in the eastern prairies 
by Andropogon scoparius is here taken by Agropyrum spicatum, its 
ecological representative. Like Andropogon, it presents the bunch 
habit in drier soils, but becomes a sod-former mth well-developed 
rhizomes under more favorable moisture conditions. Eastward, these 
prairies adjoin the main area of grassland lying east of the Rocky 
Mountains. At Missoula, Montana, for example, the Agropyrum- 
Festuca community of Washington and Idaho (Weaver, 1917) meets 
and intermingles with such eastern prairie species as Stipa comata, 
Aster multiflorus, etc., while Agropyru7n spicatum, Koeleria cristata, 
Solidago missouriensis, Achillea millefolium, and others occur through- 
out both regions. 

The writer has presented elsewhere (1917) a detailed description of 
the extent, successional relations, and floristic composition of these 
western prairies. A glance at plate 17, b and c, may give some impres- 
sion as to their general character. Hence we will proceed at once to a 
discussion of root distribution. 

THE ROOT SYSTEMS OF THE GRASSES. 

Over 60 individuals of the four dominant grasses were excavated 
and examined. Three, Koeleria cristata, Poa sandhergii, and Festuca 
ovina ingrata, are shallow-rooted, the bulk of the absorbing system 
lying above the 18-inch level, while Agropyrum spicatum penetrates to 
a maximum depth of 4 feet 10 inches. 

Agropyrum spicatum. — This is the dominant bunchgrass in eastern Wash- 
ington. It has its best development westward of the high upland prairies 
of extreme eastern Washington and along the rim-rock through the eastern 
part. The bunches are often 10 inches in diameter and reach a height of over 
3 feet. The plant blossoms in June and dries out in early July, only to take 
on renewed growth after the autumn rains and to remain green all winter. 

This grass has coarser roots than any of the other three important native 
grasses. These coarse, fibrous roots have many short laterals. Some of the 



THE PRAIRIES OF THE PACIFIC NORTHWEST. 



33 



roots reach a depth of 4 feet 10 inches, although on an average 4 feet 2 inches 
was the greatest depth attained. 

Festuca ovina ingrata. — The blue bunchgrass ranks in importance with 
Agropyrum on the well-developed high prairies west of the foothills of the 
Bitterroot Mountains between Spokane, Washington, and Lewiston, Idaho. 
Because of its abundance the very appropriate name Palouse (Fr. pelouse, a 
land clothed with a short, thick growth of herbage) was early applied to this 
region. The whole plant dries out considerably by the middle of July, but the 
autumn rains revive it and it is green throughout the rest of the year. 

Festuca ovina has a great mass of jet-black roots which occupy the soil 
thoroughly from the surface to a depth of about 18 inches, below which depth 
relatively few roots extend. None of the roots are over 1 mm. in diameter. 
They branch profusely to the third order mostly, and the laterals are usually 
less than an inch in length. This branching continues to the very tip, and 
there the laterals are usually longer. The longest root found was 3 feet 3 
inches, and the average length was 2 feet for the deepest roots, but the great 
bulk of these roots were less than 18 inches long. 

Poa sandbergii. — This species grows in small tufts, usually only from 0.5 
to 1.5 inches wide, puts out new roots when the fall rains begin, grows through- 
out the winter and spring, and evades drought by flowering late in May or in 
early June and remaining dormant the rest of the growing season. 

Poa has smaller roots than Festuca; they are more profusely branched, 
and the fine, short laterals are more numerous, smaller, and much more 
branched. The creamy-white roots spread laterally 3 to 5 inches and occupy 
thoroughly the first few inches of soil, relatively few extending below a depth 
of 8 inches, and none was found beyond 13 inches. The root branches are 
longer and more numerous at the tip than are those of Festuca. The average 
maximum depth was found to be 10 inches. 

Kceleria cristata. — This is also a dominant bunchgrass on the prairies of 
eastern Washington. It flowers in late June or early July, and like Poa 
remains dormant until revived by the autumn rains. The roots resemble 
those of Agropyrum, but taper faster and have finer laterals which branch 
mostly to the third order. These laterals, like those of the shallow-rooted 
Poa, are more numerous than in Agropyrum. The deepest root found was at 
28 inches, and 15 inches was determined as the average maximum depth. 
The shallow root habit of the three last species as compared with the deeper- 
root habit of Agropyrum and certain dicotyledonous plants is shown graph- 
ically in figures 6 and 7. 

THE ROOT SYSTEMS OF OTHER PRAIRIE SPECIES. 

Since the root systems of 21 of the most important non-grassy 
prairie species have been described in detail in a former publication 
(1915), it will be necessary here only to present a summary statement, 
so that it may be possible to compare their root systems directly with 
the plants of the eastern prairies and later with those of the plains. 
Such a comparison will help us to more clearly evaluate the response 
to the conditions under which the several plant communities grow. 

Lupinus ornatus. — This plant and L. leucophyllus are the two species of 
lupines most widely distributed on the high prairies of eastern Washington. 
Both form extensive summer societies, the former on the drier slopes and 
ridges, the latter on moist hillsides and in the valleys. Several of the 24 root 



34 THE ECOLOGICAL RELATIONS OF ROOTS. 




THE PRAIRIES OF THE PACIFIC NORTHWEST. 



35 



systems examined reached depths of over 10 feet; because of the devious 
course they pursue, the tap is frequently much longer. Indeed, one was 
found to have a length of nearly 13 feet. The abundant laterals, their wide 
spread, and the clusters of terminal branches, together with the root nodules, 
are shown in figure 7. 

Lupinus leucophyllus. — This lupine has a much larger transpiring surface 
but a much smaller absorbing surface than L. ornatus, which it resembles in 
having a well-developed tap-root, usually with several laterals, all of which 
are branched to the third and fom^th order. However, the root system is 
much less extensive than that of L. ornatus; its average depth is only about 5 
feet (fig. 7). 

Astragalus arrectus. — This legume forms marked estival societies, the 
plants drjdng up during the first week in July. It has a strongly developed 
tap-root, which usually pursues a com^e directly downward and sends out 
many strong laterals at various depths. These laterals are fairly wide- 
spreading and profusely branched to the tip. They reach depths of 4 or 5 
feet (fig. 8). 

Balsamorhiza sagittata. — The balsam-root is a dominant in the high 
prairies of eastern Washington. Its abundance, size, and duration all unite to 
make it a very important species ecologically. From the short, thick, mul- 
ticipital stem the new leaves appear in April. By the first of May the plant 
is often in full bloom, forming societies which are very conspicuous, even after 
the whole aerial part has dried up. Twenty-five root systems were excavated 
and examined. Balsamorhiza has a tap-root sometimes reaching a diameter 
of 4 inches and an extreme depth of 8 feet 10 inches. The laterals seldom 
come off in the first 6 inches of soil, but numerous strong laterals occur below 
this, sometimes 1 inch or more in diameter, and these often run rather hori- 
zontally for 2 or 3 feet before they turn downward. They may ultimately 
reach depths of 5 feet or more. The lateral branching is profuse, and in all 
directions the soil is laid hold upon. Sometimes the tap splits up into nearly 
equal parts at a depth of a few feet. The tip of the tap-root is often dead, and 
if alive is never much branched. The older part of the root especially is 
covered with a bark furrowed sometimes 0.5 inch deep. The average root 
depth was found to be 5.5 feet (fig. 6). 

Geranium viscosissimum. — This subdominant forms extensive societies in 
the estival aspect. It has a well-developed tap-root which may reach 3 inches 
in diameter. The tap sends off many laterals, both large and small, at all 
levels, all of which may branch profusely to the fifth order. The larger laterals 
usually run off in a horizontal direction, sometimes for nearly 3 feet before 
turning downward. The end of the tap-root is either unbranched or branched 
but little, and is often dead. Hard soil seems to be a marked hmiting factor 
to root growth, and under this condition, especially, the usual very irregular 
course of descent is greatly emphasized. Twenty-four root systems were 
examined. While one plant penetrated to 9.5 feet, the average depth was 
found to be 5.5 feet (fig. 6). 

Wyethia amplexicaulis. — This large, rather coarse plant is at home on 
moist hiUsides and especially in valleys, where it forms extensive estival 
societies. It has a fleshy tap-root, which sometimes measures 9 inches in 
circumference and may reach a depth of 6 feet 5 inches. It usually has several 
strong laterals which come off from 8 inches to 3 feet in depth, and may run 
out in a somewhat horizontal direction for 3 or 4 feet from the main root. 
Often at a depth of 1 to 3 feet the whole tap breaks up into 2 to 5 nearly equal 



THE PRAIRIES OF THE PACIFIC NORTHWEST. 



37 



parts which pursue a downward course, or later extend out as laterals. Pri- 
mary laterals are not much branched. The w^hole root from crown to near the 
tip is covered with scattered small laterals, usually not over 2 mm. in diameter. 
These are poorly branched, not often giving off roots of the fourth order. The 
tip of the main root hkewise is Httle branched. The roots dry out and shrink 
rapidly upon removal from the soil. The 18 plants examined had an average 
root depth of 5 feet (fig. 7). 

Heuchera glabella. — This saxifrage has a strong tap-root supplied, especially 
in the first foot of soil, with rather numerous but poorly branched laterals. 
It reaches depths of about 5 feet (fig. 8). 

Leptotsenia miiltifida. — Leptotcenia is a subdominant which forms con- 
spicuous societies in the vernal aspect. It has a large fleshy tap-root, some- 
times 7 inches in circumference, wiiich may reach a depth of over 5 feet. The 
fusiform roots may narrow down gradually or rather abruptly, even to a 
diameter of 1 or 2 mm., and then again enlarge to a size equaling the original. 
This is sometimes repeated several times, thus giving the root as a whole a 
beaded appearance. In general the roots are very poorly branched. The 
average depth of penetration is 5 feet (fig. 7) . 

HeliantheUa douglasil — This species, Hke the following, forms societies in 
the autumnal aspect. It has a tap-root with a diameter seldom more than 2 
cm. It throws out many large laterals just beneath the surface, most of 
which come off within the first 18 inches of soil, although there are some 
lower, and a cone may be formed aU the way to the tip. The laterals usually 
pursue rather a vertically downward course and are profusely branched. Of 
the 20 plants examined the deepest penetrated only to 5.5 feet, while the aver- 
age depth was found to be 4.5 feet (fig. 8). 

Hoorebekia racemosa. — The strong tap-root of this plant sometimes reaches 
a depth of 10 or 11 feet, but usually ends near the 5.5 foot level. The larger 
laterals, if any, are often thro-^m off within the first 18 inches of soil. These 
are branched to the third order and may run off in a rather horizontal direc- 
tion. The tap grows directly downward and is sparingly branched aU the 
wav to the tip. The main root is usually not more than 0.5 inch in diameter 
(fig. 6). 

Lithospermiun ruderale. — The tap-root of this species varies from 3 to 10 
inches in circumference and gives promise of a deeper root system than is 
actually attained. Only one plant reached a depth of over 6 feet, while the 
average depth of the 5 specimens examined was 4 feet 10 inches. Large 
laterals were sent off from the main root at various depths from 1 to 2 feet. 
These ran horizontally several feet before turning downward. 

Sieversia ciliata. — This species forms societies in the estiva! aspect. Few 
roots of Sieversia penetrate beyond depths of 5 feet 6 inches. It sends out as 
many as 20 to 30 roots from a single inch of its thick rootstock. None of these 
roots is over 3 to 4 mm. in diameter. They pursue a vertically downward 
course and branch profusely all the way to the tip, sending off laterals seldom 
over 3 inches long but branched to the fifth order. The 11 plants examined 
gave an average root depth of 4 feet 9 inches (fig. 7) . 

Sidalcea oregana. — This mallow is confined to north hillsides and low 
ground. It has a tap-root which gives off strong laterals, beginning in the 
first foot of soil. The tap penetrates to depths of only 3 or 4 feet. The roots 
are characterized by the origin of short laterals in groups of 3 to 8 (fig. 8) . 



38 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Hieracium scouleri. — Thishawkweed is one of the few species of the western 
prairies which forms autumnal societies. For the 26 root systems examined 
an average depth of 5.5 feet was determined. It sends out numerous roots, 
as many as 50 from a single inch of its rhizome. The rhizome is 8 to 12 inches 
long. None of these roots are over 3 mm. in diameter. They pursue a nearly 
vertically downward course and throw off practically no laterals, except where 
they enter earthworm holes. Here strong laterals, equahng the main root in 
diameter, are developed and run parallel with it in the hole a foot or two or 
further. All give rise to abundant root-hairs and it is seldom that these roots 
again enter the soil. The tip of the main root may branch very profusely, or 
very little if in hard soil. The longest root reached a depth of 7 feet 9 inches, 
although 5 feet 4 inches was found to be the average depth (fig. 6) . 

Potentilla blaschkeana. — The tufted stems of this subdominant are borne 
on a short, thick crown from which 2 to 5 main roots originate. These average 
about 7 mm. in diameter. They taper off gradually till at about 2 feet in 
depth they are often only 2 mm. in diameter. Here they usually branch 
dichotomously, and again branching, break up into numerous small roots — 
mere hairs — ^which have a strong tendency to grow in earthworm holes. 
These they follow for perhaps 2 feet before entering the soil again. In these 
holes they give off many threadlike branches which follow down the same 
hole, branching profusely and often not reentering the soil. The roots are 
dark brown in color. They are unbranched or very poorly branched at the tip. 
Thirty root systems were examined. The longest roots penetrated to a depth 
of 7.5 feet, while the average root depth was found to be about 5 feet (fig. 6). 

Eriogonum heracleoides. — This mat-former, at home on dry ridges, has a 
strong, woody tap-root often an inch in diameter. It sometimes breaks up 
just below the soil surface into several more or less equal parts. The roots 
penetrate to a depth of 6 to 8 feet or more. They spread widely and are well 
branched. 

Some examinations were also made of the roots of Rosa nutkana and Sym- 
phoricarpos racemosus, both very common in nearly all prairie situations. 
They were found extending to depths of from 6 to over 8 feet. 7ns mis- 
souriensis roots penetrated the soils of dry hillsides to 3 feet 10 inches, and 
Berheris repens to a depth of over 10 feet. 

PRAIRIE ROOT SYSTEMS AND PRAIRIE ENVIRONMENT. 

From these data it may readily be seen that most of the prairie 
plants are deep-rooted. With the exception of Kceleria cristata, Poa 
sandbergii, and Festuca ovina (which are confined largely to the surface 
18 inches of soil) all of the species examined reached depths of from 4 
to 6 feet, while some penetrated even deeper. However, when com- 
pared with the species of the eastern prairie, these plants are not as 
deeply rooted. It may be recalled (p. 18) that about 55 per cent of 
the roots in the latter conununity penetrated well beyond 5 feet and 
many of them beyond 10 or 12 feet. Here, however, with a few 
exceptions, the plants usually terminate their root systems at depths 
of about 5 or 6 feet, while only one or two species have roots penetrating 
to 10 feet. A proper interpretation of this difference can be made only 
by a careful study of the habitat. 



THE PRAIRIES OF THE PACIFIC NORTHWEST. 39 



The prairies of southeastern Washington and their eastward exten- 
sion into adjacent Idaho occupy a position between the foothills of the 
Bitterroot Mountains on the east and the sagebrush region of western 
Adams, eastern Franklin, and western Walla Walla counties of Wash- 
ington on the west. On the south they are bounded by that high 
upfold of the lava-rock known as the Blue Mountains. Northward the 
Spokane gravels, extending somewhat southward of Spokane, with 
their open growth of yellow pine, mark at the same time the general 
northern boundary of the exposed part of the great lava sheet and its 
accompanying prairie formation. Since the supply of water in this 
region is the chief hmiting factor to plant growth, we shall first con- 
sider the total amount of precipitation, with its seasonal distribution, 
after which the water-content of the soil will be considered. 

Hemmed in on all sides by mountains, and especially cut off from 
the moist winds of the Pacific by the Cascades, the Columbian Plateau 
has a low annual precipitation. Even on its high eastern border, 
where these studies were carried on, it is only 21 inches. Table 7 
shows in inches the mean monthly and annual precipitation at Pull- 
man, Washington. 

Table 7. 



Month. 


Precipi- 
tation. 


Month. 


Precipi- 
tation. 


Feb 

May 

July 


inches. 
2.55 
2.18 
2.02 
1.50 
1.84 
1.20 
0.57 


Aug 

Sept. 

Oct 

Dec 

Total . . 


inches. 
0.68 
1.29 
1.70 
3.41 
2.66 

21.60 



It may be seen at a glance that about three-fourths of the precipita- 
tion occurs during the resting period. The light showers of July and 
August seldom have much influence on the water-content of the soil. 
The soils of this region may be compared to a gigantic reservoir 
replenished mostly during the resting season and rather thoroughly 
emptied of its water during the summer. It is not the absolute rainfall 
figures alone which furnish a criterion of climate, for the maximum 
duration of the drought period constitutes a limiting factor of the 
greatest importance. The great problem is the extent to which soil 
water derived from the winter precipitation is conserved through the 
weeks of drought. The rains in southeastern Washington are so gentle 
that there is practically no run-off, and the silt-loam soils have a 
wonderfully retentive power. 

The prairie soil has originated from the decomposed underlying 
basalt. It consists of a friable, dark-brown silt loam which has a water- 



40 



THE ECOLOGICAL RELATIONS OF ROOTS. 



holding capacity of 50 or 60 per cent of its dry weight. By the action 
of water and especially of the prevaihng southwest wind, the prairie 
topography has been molded into rounded hills which reach a height 
of 100 to 360 feet and resemble sand-dunes. The soil is usually many 



Table 8. 





SW. slope. 


NE. slope. 




SW SloDP 


NF, si on A 


Date. 


Wilting 


Wiltina- 


Date. 


Wil+ino' 


Wil+ino' 

T T XI UXXX^ 












OUcillCxolXb 


1913. 






1913. 






Apr. 25 


10.0 


21.0 


Aug. 15 ... . 


-4.2 


0.0 


May 2 


12.0 


22.2 


Aug. 22 


—2.6 


— 1.3 


May 8 


9.0 


20.5 


Aug. 28 


-3.0 


-1.0 


May 14 ... . 


13.3 


25.6 


Sept. 5 


-2.2 


-2.0 


May 20 


13.0 


25.0 


Sept. 10 


-3.4 


-1.8 


May 27 


5.2 


18.0 


Sept. 17 


-3.9 


-1.0 


J\ine 2 . . . . 


2.0 


16.6 


Sept. 25. . . . 


-2.2 


0.7 


June 10 ... . 


4.8 


21.2 


1914. 






June 15 ... . 


5.0 


17.4 


May 22 


4.0 


13.0 


June 25 ... . 


8.6 


26.0 


June 2 


0.1 


7.5 


July 2 


4.6 


19.0 


June 26 


-1.3 


1.3 


July 8.... 


4.0 


15.2 


July 4.... 


-0.2 


1.8 


July 16.... 


-0.5 


9.4 


July 8.... 


-1.2 


1.1 


July 21.... 


-0.5 


6.4 


July 19.... 


-4.5 


0.0 


July 28.... 


-2.6 


4.4 


July 27.... 


-5.0 


-2.0 


Aug. 4 . . . . 


-1.4 


0.2 


Aug. 3 


-4.0 


-2.0 


Aug. 9 


-3.0 


0.0 


Aug. 10 


-4.0 


-2.8 



Table 9. 
Southwest Slope. 



Date. 


Wilting coefl&cients at stated depths. 


11.5 at 
to 6 inches. 


11.0 at 
6 to 12 inches. 


14.2 at 
2 feet. 


13.5 at 
3 feet. 


13.5 at 
4 feet. 


14.0 at 
5 feet. 


Dec. 13, 1913 
Apr. 18,1914... 
June 3, 1914 . . . 
July 6,1914... 
Aug. 15, 1914... 


14.5 
-1.0 
-2.5 
-4.5 


11.5 
13.5 
1.4 
0.0 
-1.6 


-1.2 
8.1 
3.4 

-0.7 
1.9 


-0.3 
9.7 
7.3 
1.0 
0.6 


0.5 
8.2 
8.1 
5.1 
2.1 


0.5 
5.5 
8.0 
6.8 
2.6 


Northeast Slope. 


Date. 


Wilting coefficients at stated depths. 


12.2 at 
to 6 inches. 


12.2 at 
6 to 12 inches. 


13.4 at 
2 feet. 


13.6 at 
3 feet. 


14.2 at 
4 feet. 


14.0 at 
5 feet. 


Dec. 6, 1913 
Apr. 18, 1914... 
June 3,1914... 
July 6,1914... 
Aug. 15, 1914... 


22.8 
6.6 
3.3 

-3.0 


15.8 
20.0 
8.5 
1.0 
-1.7 


11.2 
15.6 
13.1 
6.8 
-0.6 


-0.8 
13.4 
16.5 
12.1 
0.0 


0.6 
9.9 
13.6 
10.8 
2.3 


5.0 
9.5 
9.8 
3.9 
3.0 



THE PRAIRIES OF THE PACIFIC NORTHWEST. 



41 



feet deep. The wind has drifted much surface soil and humus material 
from the exposed south and southwest slopes and deposited it upon 
the steeper north and northeast leeward slopes. 

It may be seen in tables 8 and 9 that the soil in the spring shows a 
maximum water-content, the autumn and winter precipitation having 
replenished the water lost during the long period of summer drought. 
Table 8 gives the available water-content of the prairie soil to a depth 
of 10 inches during 1913 and 1914. Table 9 shows the available water- 
content of prairie soils to a depth of 5 feet during 1913-14. The minus 
sign indicates water non-available for plant growth. 



Table 10. 



Depth. 


May 22. 


July 6. 


Aug. 15. 


NW. 


S. 


NW. 


S. 


NW. 


S. 




23.4 


12.5 


15.2 


11.1 


11.6 


6.7 


6 to 12 inches 


25.7 


17.9 


13.7 


11.4 


12.1 


10.2 


At 2 feet 


27.0 


19.9 


18.9 


12.6 


11.8 


10.2 


At 3 feet 


30.2 


20.2 


23.7 


13.2 


14.4 


10.6 


At 4 feet 


27.2 


22.0 


24.9 


17.3 


18.0 


10.5 




24.3 


22.4 


21.3 


19.6 


19.5 


12.3 



The greater water-content on northerly exposed slopes is the result 
of several causes. More precipitation actually occurs here as far as 
effective moisture is concerned, much of the 46 inches of normal snow- 
fall being blown over to the sheltered north hillsides. The soil on 
north slopes is deeper, has more humus, and a greater water-holding 
capacity. It has been seen that this reflects itself both in plant dis- 
tribution and root development. An examination of table 9 shows 
not only a gradual depletion of soil moisture as the season progresses, 
but also that to a depth of 2 or 3 feet little or no moisture is available 
in late summer. 

This seasonal march of soil-moisture, together with its greater 
scarcity on south slopes, is further illustrated in table 10, which shows 
the water-content of soil samples taken from another set of stations 
during 1914. 

To evade the drought conditions thus imposed upon them, a few 
plants, notably the shallow-rooted grasses, complete their period of 
growth and produce seed early in the season, and then lie dormant 
until revived by the autumn rains. Most prairie species, however, 
extend their roots far down into the deeper moist soil. As the moisture 
supply decreases with the progress of summer, the evaporating power 
of the air becomes more and more intensive. This is shown in table 11. 

Thus, high evaporation and low available soil moisture combine 
to make conditions rather unfavorable for plant growth, especially 



42 



THE ECOLOGICAL RELATIONS OF ROOTS. 



in late summer and in exposed situations. As has already been men- 
tioned, late-maturing species are not at all abundant. No late- 
maturing grasses are components of the prairie flora. Flowering 
begins later and growth continues much longer on north hillsides. 

Soil temperatures on the two exposures differ notably. These data 
are shown in table 12, which gives the soil temperature at 1 to 5 feet 
on April 18 and August 15, 1914, on a northeast and southwest slope 
respectively. 



Table 11. — Average daily evaporation in cubic centimeters on a northeast 
and a southwest slope respectively. 



Date. 


SW. slope. 


NE. slope. 


Date. 


SW. slope. 


NE. slope. 


1913. 






1913. 






May 


10. . 


12.0 


10.5 


Sept. 


3. . 


33.0 


21.7 


May 


15. . 


9.8 


6.5 


Sept. 


7. . 


32.1 


21.6 


May 


26. . 


13.1 


10.2 


Sept. 


13. . 


33.6 


23.8 


June 


3. . 


15.2 


10.2 


Sept. 


20. . 


46.0 


20.0 


June 


10. . 


13.0 


8.6 


Sept. 


23. . 


21.0 


17.0 


June 


15.. 


14.2 


7.8 


1914. 






June 


25. . 


7.8 


5.4 


May 


15. . 


20.1 


19.0 


July 


3. . 


11.0 


6.7 


May 


21. . 


19.0 


15.1 


July 


8. . 


21.0 


13.5 


May 


30. . 


17.8 


15.6 


July 


13.. 


27.2 


14.0 


June 


7. . 


20.0 


11.2 


July 


18. . 


31.0 


21.8 


June 


15. . 


16.1 


12.5 


July 


25. . 


30.8 


20.3 


June 


22. . 


32.1 


19.0 


July 


29. . 


41.5 


26.4 


June 


29. . 


19.0 


12.8 


Aug. 


4. . 


33.0 


23.8 


July 


6. . 


27.2 


22.5 


Aug. 


10. . 


39.5 


24.5 


July 


13. . 


44.0 


30.5 


Aug. 


16. . 


26.5 


15.8 


July 


20. . 


44.8 


26.0 


Aug. 


20. . 


38.0 


27.2 


Jvily 


27. . 


49.0 


32.0 


Aug. 


25. . 


44.3 


30.0 


Aug. 


3. . 


51.5 


42.6 


Aug. 


31. . 


43.8 


27.5 


Aug. 


10. . 


57.8 


39.1 










Aug. 


15. . 


64.0 


50.1 



Table 12. 



Depth. 


April 18. 


August 15. 


NE. 


SW. 


NE. 


SW. 


1 foot. . . . 

2 feet 

3 feet.... 

4 feet.... 

5 feet 


45.7°F. 

45.1 

45.0 

45.0 

45.0 


50.3°F. 

50.0 

48.2 

47.0 

46.4 


65.0°F. 

61.7 

58.0 

57.2 

54.5 


73.0°F. 

70.0 

66.2 

64.4 

62.6 



The daily midsummer range at a depth of 3 inches on the northeast 
and southwest slopes was 12° F. and 26° F. respectively. It is probable 
that these differences in temperature have considerable effect at least 
on the rate of root development (Cannon, 1918). 

Summarizing briefly the factors of the habitat in which these 
studies were carried on, we find a region of moderate winter and low 



THE PRAIRIES OF THE PACIFIC NORTHWEST. 



43 



summer precipitation. The soils are composed of a fine silt loam of 
high water-holding capacity and they are usually very deep. In early 
summer the superficial layers of soil soon lose all of their water avail- 
able for plant growth, and as the season advances this condition occurs 
in the deeper soils, while the entire soil-mass (to a depth of 5 feet and 
beyond) gradually yields most of its available water. Soil temperatures 
at 3 inches show a daily range of from 3° F. to 24° F., while at 1 foot 
the daily range is seldom over 1° F. The seasonal range (April to 
August) of the soil temperatures varies from 22° F. at 1 foot to 16° F. 
at 5 feet. Air temperatures show a mean daily range varying from 
about 25° F. in April and May to 38° F. in July and August. The 
cool nights on the high plateau tend to counteract the low humidity of 
the day and to reduce the high daily rates of evaporation. 

North and northeast slopes are less xerophytic than the south and 
southwest slopes. This is due in part to actually greater precipita- 
tion caused by blowing snow and in part to soil texture, which is more 
open, has more humus, and a greater water-holding capacity. These 
factors are reflected in the greater amount of soil-water and in lower 
soil temperatures. Likewise these slopes are sheltered from the drying 
southwest vnnds and from the perpendicular rays of the sun. This 
is reflected in slightly lower air temperatures and greater humidity, 
and especially in the lower evaporating power of the air. On the part 
of the plants the most obvious response to these severe environmental 
conditions is the development of extensive root systems. For just as 
the evaporating power of the air and the nature of the transpiring 
organs determine the water-loss of plants, likewise the soil-water 
and the nature of the root systems determine the supply. 

Table 13. — Average root depths (in inches) of plants on different slopes. 



Lupinus ornatus 

Lithospermum ruderale. . 
Potentilla blaschkeana. . . 
Geranium viscosissimum. 

Hoorebekia racemosa 

Hieracium scouleri 

Helianthella douglasii . . . 
Balsamorhiza sagittata . . 
Leptotaenia multifida. . . . 
Agropyrum spicatum 



SW. slope. 



72.1 



64.0 
62.8 
52.0 
61.5 



NE. slope. 



71.0 



48, 
59, 
56, 
63, 
63. 
55. 



54.7 
49.6 



NW. slope. 



128.4 
74.5 
64.3 
72.2 
68.2 
66.8 
55.1 
69.8 
47.3 
38.6 



It is instructive to note that according to the types of root systems 
as set forth by Cannon, all the roots here described, with the possible 
exception of Leptotcmia multifida, fall under the generalized class. This 
was true also for all but a few of the eastern prairie species. By a 



44 



THE ECOLOGICAL RELATIONS OF ROOTS. 



generalized root system is meant one that has both the tap and the 
laterals well developed. They penetrate deeply and reach out widely. 
In contrast, the specialized root system has either the tap-root as the 
chief feature or the laterals are placed near the surface and are espe- 
cially well developed, as in cacti. The generalized type of root is much 
more plastic and consequently reacts to a wider range of conditions 
than does the specialized type. 

In considering the question of the susceptibiUty of roots to modi- 
fication through variation in the soil texture or its water-content, as 
against the conservative inherited tendencies, table 13 is instructive. 

It appears that the root lengths on northeast and southwest slopes 
are about the same and that the marked environmental differences 
play little part in determining root depth. The greater root depth of 
most species on the northwest slope at a station which combined a 
porous moist soil with rather extreme xerophytic above-ground con- 
ditions is at once evident. In all habitats studied the writer has found 
that hard soil profoundly affects the amount of branching, laterals 
practically always being more numerous in a less compact substratum. 
This may be closely connected with water-content and aeration — 
the looser soils, of course, if of the same type, having the greater water- 
holding capacity. A discu^ion of the effects of various habitats on 
the root development of polydemics will be found on page 110. 

The roots of several species showed a marked increase in their out- 
put of branches upon leaving the compact soil and entering earth- 
worm burrows. In practically all cases the root-tips under such con- 
ditions were alive and at least well covered with root-hairs, while in 
the more compact soils of dry slopes especially the root-tips were 
often dead and decayed. The cause of these differences is yet to be 
determined. The differences may be due to the mechanical resistance 
offered by the soils, or to changed conditions of aeration, or perhaps 
to both factors acting together. Cannon and Free (1917) have found 
that roots of various plants respond quite differently to variations 
in the composition of the atmosphere. This difference appears to be 
related to the character of the natural habitat of the species in ques- 
tion. Their results indicate that plants growing in well-drained soil 
are much more sensitive to the composition of the soil atmosphere 
than those from poorly drained and poorly aerated habitats. 

Contrasting grassland species of the east and west, we reach the 
following conclusions: Prairie plants of eastern Nebraska growing in 
the clay-loam or loess soil under an annual precipitation of 28 to 30 
inches, most of which occurs during the growing season, and with an 
average daily summer evaporation of 21 c.c, have root systems very 
similar in distribution but somewhat greater in depth than prairie 
species of southeastern Washington growing in the silt-loam of dis- 



THE PRAIRIES OF THE PACIFIC NORTHWEST. 



45 



integrated basalt under an annual precipitation of 21 inches, most of 
which falls during the resting season, and with an average daily sum- 
mer evaporation of 30 c.c. 

In both of these semiarid regions it has been shown that the water- 
content of the soil is reduced to the non-available point to a depth of 
3 or 4 feet, at least during certain years. It has also been shown that 
in both of these prairie communities all of the dicotyledonous plants, 
as well as many of the grasses, extend their root systems to depths of 
from 3 to 7 feet or more. 

The absence of very shallow, widely spreading laterals is charac- 
teristic of plants of both prairie communities, perhaps being more 
pronounced in the Palouse region. This is in distinct contrast to the 
root habit of many plains species, to which we shall now turn our 
attention. 



46 



THE ECOLOGICAL RELATIONS OF ROOTS. 



IV. THE PLAINS ASSOCIATION. 

This great grassland community extends from northwestern Texas 
and northern New Mexico to northern Colorado and Nebraska and 
from the Rocky Mountains eastward to central Texas and Kansas, 
where it meets the prairie. While soil moisture seems to be the limit- 
ing factor in the extension of the prairies westward, this is so inti- 
mately connected with the amount and distribution of the precipita- 
tion and with the soil type that great tongues of true prairie extend 
far into the Great Plains. This is especially true northward, while over 
much of the broad ecotone plains and prairie vegetation alternate. 
The plains community differs from the prairie chiefly in the domi- 
nance of short grasses, especially Bouteloua gracilis and Bulhilis dacty- 
hides, and in the smaller number and reduced importance of the societies. 

An excellent description of various plains communities, together 
with theii' successional relations and indicator values, especially for 
Colorado, has been given by Shantz (1911). Plate 18, a, affords a 
ghmpse of the plains vegetation near Colorado Springs, Colorado, 
where the roots described below were excavated. Here about 80 to 
85 per cent of the soil surface was covered with vegetation. 

Bouteloua gracilis. — ^This well-known and important grass is a dominant 
over the plains from western Nebraska to the Rocky Mountains and from 
Texas far into Canada. From the standpoint of grazing it ranks among the 
highest of all the grasses, being equaled only by Bulhilis dadyloides. 

The soil is well filled with fine rootlets to a depth of 30 inches, while in the 
next 6 inches they are still fairly abundant, some of the longer ones penetrating 
to a maximum depth of 48 inches. The surface roots spread very widely in 
the shallow soil to 1.5 feet or more and are exceedingly well branched. 

Aristida purpurea. — Much of the plains association is characterized by the 
bunches of this very widely distributed dominant, wherever overgrazing or 
other disturbance has given it a foothold in competition with Bouteloua gracilis. 
From the bases of the individual clumps strong fibrous roots arise, from 0.5 to 
1 mm. in diameter. These rather coarse roots either descend vertically or run 
off obliquely at an angle of from 20 to more than 45 degrees with the surface 
of the soil to a distance of 5 to 8 inches before turning downward. The sur- 
face of the soil below the clump is completely filled with these cord-like roots, 
but they do not branch profusely until they have penetrated 4 or 5 inches into 
the soil. Below this depth for 3 or 4 feet the roots become smaller, giving off 
both large and small laterals, many of the latter being only 1 to 10 mm. in 
length. The ground is rather well occupied to a depth of 3 feet, at which 
depth many of the roots terminate in rather well-branched tips, while others 
penetrate to a depth of 4 feet or slightly beyond. A maximum depth of 4 feet 
3 inches was recorded for several of these tiny, hairlike termini. Branching is 
especially well developed in joints between the lumps of hard soil. The older 
roots are ashy-gray in color, and are provided with a rather papery cortex, 
which upon removal reveals the tough, light-yellow stele. The deeper roots 
are threadlike and are very fragile and usually well branched, although they 
sometimes run for several inches without giving off any laterals (fig. 9). 
Fissures occur in the soil, extending to a depth of 2 or 3 feet. These have 



THE PLAINS ASSOCIATION. 



47 



e\ddently been open cracks and the soil on the sides of them is much darker 
in color, having been washed down from the sm'face. It is in these crevices, 
where the moisture-content is somewhat higher, that grasses and other plants 
branch and rebranch so profusely. 




Fig. 9. — Aristida purpurea. Fig. 10. — Artemisia frigida. 

MuWenbergia gracilliina. — This grass is also a dominant of the short-grass 
plains; it is less widely spread and less abundant than Bouteloua gracilis, 
though in some places it ranks close to the latter. Superficially, it appears like 
Bulbilis dadyloides, because of its mat-like growth in small areas and the short, 
curled leaves. Frequently the center of the mat is dead and only the 
peripheral portions produce flowers. 

A trench over 4 feet long was dug with, one face cutting through a pure 
stand of this grass to a depth of about 5 feet. Great clusters of roots, only 
about 0.5 mm. or less in diameter, ran off in all directions from the very sur- 



48 



THE ECOLOGICAL RELATIONS OF BOOTS. 




THE PLAINS ASSOCIATION. 



49 



face to the average working depth, and are abundantly supplied with short, 
much rebranched laterals. Many of these spread very widely laterally, 
branching and rebranching into both long and short termini, so that the 
surface 27 inches of soil is completely filled with a dense network of absorbing 
rootlets. Every cubic centimeter of the soil to a depth of 2 feet is occupied by 
a network of this absorbing system. At a depth of 27 inches most of the root- 
lets end in very much branched termini, but a few penetrate to greater dis- 
tances. By undercutting the face of the trench and working upward from a 
depth of 5 feet, several of these threadlike rootlets were encountered at a 
depth of 55 inches. The roots are tan in color. 

Gutierrezia sarothrse. — ^This half-shrub occurs throughout the plains region, 
although it is usually more abundant in areas of less thoroughly disintegrated 
soil or in overgrazed areas. In such situations it forms extensive societies. 
The plants examined were quite abundant and from 6 to 9 inches high. Nu- 
merous stems arise from a strong tap-root from 5 to 8 mm. in diameter. The 
tap immediately gives rise to 5 to 9 large laterals, 2 or 3 mm. in diameter, and 
as a result, at a depth of 2 to 6 inches the tap is no larger than one of these 
laterals. The main root descends more or less vertically and at a foot in depth 
is seldom over 1 mm. in diameter. It is much curved and kinked, but these 
twists are usually small, so that at a maximum depth of 6 or 6.5 feet the tip 
of the root is not more than a foot or 18 inches horizontally from the base of 
the stem. Just at the surface, to a depth of 12 to 18 inches, very numerous 
threadlike laterals, 0.5 mm. or less in diameter, run off horizontally in all 
directions for a distance of 3 to 24 inches, the plant thus being well supplied 
with surface feeders. The large laterals frequently pursue a course nearly 
parallel with the surface of the soil at a depth of 2 to 5 inches, or run some- 
what obliquely to distances of 12 to 20 inches before turning rather abruptly 
and vertically downward. Like the tap, they are profusely branched and 
rebranched with short, threadlike laterals, but after a depth of 18 inches is 
reached the branching becomes much poorer. These deeper roots may run a 
distance of a foot or more without giving off any laterals, or again short laterals 
only 2 or 3 inches long may occur in groups. The tips of these deeper roots 
are only fairly well branched. Some were found which gave off a whole net- 
work of branches matted in the cracks. While the larger roots are brown, 
tough, and rather woody, the deeper ones are nearly white and rather brittle. 
Several roots were traced to a depth of over 5 feet, while a few reached a 
maximum depth of 6 to 6.5 feet (fig. 11). 

Psoralea tenuiflora. — ^This is one of the important legumes throughout the 
plains. Under favorable conditions of soil moisture it forms extensive societies. 
A number of individuals of this species were examined. They all had strong 
tap-roots varying in diameter with the age of the plant. One large root, 
which unfortunately was dead, had a diameter of 2 cm. At a depth of 1.5 feet 
it gave off numerous large lateral branches, the branching system being not 
unlike that of the more robust form of the prairies. A younger specimen, 
with a tap-root 7 mm. in diameter, pursued an almost vertically downward 
course to a depth of 42 inches. At a depth of 2 feet a small lateral ran off 
rather horizontally to a distance of 20 inches. No other branching occurred 
except near the tip, where 5 large branches and several smaller ones originated. 
Except for a very few rootlets, 5 to 10 mm. long and distributed very irregularly 
along the course of the tap, no other branching occurred. 

A large plant, with a tap 8 mm. in diameter, reached a depth of 8 feet. 
Like the former, it pursued an almost vertically downward course. It was 
unbranched to a depth of 28 inches, where it gave off a horizontal lateral only 



50 



THE ECOLOGICAL RELATIONS OF ROOTS. 



1.5 mm. in diameter and 27 inches long. A similar lateral about the same 
size arose 6 inches deeper in the soil. At 5 feet the tap, now only 1.5 mm. in 
diameter, forked dichotomously. At 6 feet the two branches entered a layer 
of moist sand through which they ran for 26 inches to a total depth of 8 feet, 
but they gave off very few branches. Rootlets of other plants of this species 
were quite numerous in these deeper soils. Finally, a plant was excavated 
and photographed (plate 18, b) which reached a depth of 12 feet 2 inches. 
The roots are brown to reddish-brown in color. 

Artemisia frigida. — Mountain sage is a competitor of the plains grasses, 
especially northward, for the rather meager water-supply. Near the moun- 
tains, in rocky or gravelly situations, it frequently forms extensive societies. 

From the base of the clustered woody stems a tap-root arises; it is from 5 
to about 10 mm. in diameter and descends rather vertically to a distance of 
from 4 to over 6 feet. The tap, however, becomes rapidly attenuated, so that 
at a depth of 6 inches it is scarcely 1 to 2 mm. in diameter and indeed no 
larger than the numerous strong, rather horizontal laterals which arise from 
the tap or the base of the woody stem at just below the ground surface. These 
larger laterals, frequently as many as 5 to 9 on a single plant, run off obliquely 
in the shallow soil, sometimes ahnost horizontally and from a depth of only 
1 to 2 inches to a distance of 8 to 12 inches or more before turning rather 
abruptly downward. The woody bases of the stem, as well as the first 6 to 10 
inches of tap-root, give rise to very abundant rootlets, 1 nam. or less in diameter, 
which thoroughly fill the surface soil, branching and rebranching into thread- 
like termini and furnishing Artemisia, as is the case with so many plains plants, 
with a splendid surface absorbing system. 

Like the tap-root, the larger laterals upon turning downward become 
greatly attenuated, the diameter remaining imiformly a millimeter or less for 
several feet. When near the ends they often become hairlike but are not well 
branched. The deeper roots, while branching from time to time and pursuing 
a rather tortuous course through the soil, are characterized by the absence of 
numerous laterals. A large number of plants were examined and most of the 
major branches extended with the tap to depths of from 3 to 6 feet. The 
deepest root examined entered at a depth of 5 feet into the soft soil filling an 
ancient burrow, in which it continued, giving rise from time to time to long, 
threadlike, unbranched laterals. It reached a depth of 7 feet 9 inches. The 
roots are chestnut-brown in color and all but the larger ones are very brittle, 
being removed with extreme difficulty from the hard, lumpy soil (fig. 10). 

Argemone platyceras. — This wild poppy, conspicuous because of its large 
size and showy white flowers, is found as a frequent component of plains 
vegetation throughout much of the association. As a rule, it is more abundant 
in disturbed areas, often becoming ruderal. 

The plant examined had a strong tap-root. It was partially decayed, por- 
tions of it being dead and easily dug out. At the end of the first foot, however, 
it branched dichotomously. These branches diverged not more than 6 to 8 
inches and took a downward course, continuing to branch dichotomously at 
irregular intervals for the first 8 feet, the whole trend being downward (fig. 
13). At no point in the first 8 feet of soil did the branches spread more than 
1.5 feet from the vertical. At about 8 feet a layer of moist sand was encoun- 
tered and here some of the branches turned outward almost horizontally, 
extending 18 to 24 inches, where they ended. Other branches continued 
downward through moist sand to a maximum depth of over 12 feet, the last 
10 to 12 inches of their length passing into a very moist sandy clay. From 



THE PLAINS ASSOCIATION. 



51 



about 4 feet downward these branches gave off occasional laterals about 2 to 
4 mm. in diameter, which extended from a few inches to a foot or more. These 
laterals tended to take a more or less horizontal direction. 

The color of the root is dark brown. It was very thick and fleshy and 
extremely brittle, so much so that near the terminus great difficulty was 
found in following it. Throughout its length, whenever broken, a clear to 
milky fluid exuded in limited quantities. The surface of the root was extremely 
rough and pitted; its diameter was often irregular. Throughout the whole 
course two or more branches had a tendency to twist about each other. This 
was marked in branches occurring not more than 2 feet from the surface, 
again at intermediate depths, and still again at not less than 6 inches from the 
extreme depth. 

Yucca glauca. — ^The soap-weed is a widely distributed species, being 
especially abundant on dry, sandy, rocky slopes and forming extensive 
societies throughout many areas in the plains association. Its size, duration, 
and often its abundance combine to make it an important species. From the 
standpoint of the stockman it is often a bad weed. 

A number of specimens were examined. An excavation was made about 
two large plants growing 2 feet apart. They each sent down a strong caudex 
3 inches in diameter, but neither of them reached a depth greater than 18 
inches. At this depth they branched and ran off laterally in a direction either 
parallel with or ascending toward the surface. The two plants were connected 
by a large underground rootstock (plate 17, a), while another (over 2 inches 
in diameter) ran off at nearly right angles from the connecting rootstock and 
reached the surface 3 feet beyond. It had given rise to several small plantlets. 
From the multicipital stem as many as 25 to more than 50 rosettes of leaves 
arose. Many of these were dead. One crown measured 21 by 26 inches in 
diameter, this being only a medium-sized plant. It reached a height of 30 
inches. The multicipital branching extended to a depth of 5 to 7 inches and 
profound contractions were in evidence. 

Beginning at the very surface, the stems are supplied profusely with roots. 
These vary from 2 to 4 mm. in diameter and run off horizontally to great 
distances, the lateral spread being remarkable. A number of these hori- 
zontal roots were traced to a distance of 32 feet (fig. 12). They occupied the 
soil chiefly at a depth of 6 to 18 inches, and, because of a somewhat tortuous 
course, terminated at a horizontal distance of 27 feet from the caudex from 
which they arose. These roots are reddish in color, of nearly uniform diameter 
for long distances, and very sparingly branched. Only at intervals do short, 
usually unbranched, secondary laterals occur. The cortex is thick and fleshy 
and serves for water storage; upon the death of the older roots it shreds off 
and reveals the tough wiry stele. The abundance of these roots is surprising. 
In a single square foot of vertical trench face, at a depth of 18 inches, 54 roots 
were counted. They extend outward in all directions from the rhizome, as 
was proven by digging a series of small trenches at distances of 18 to 20 feet 
on all sides of isolated specimens. Invariably Yucca roots were found. Such 
a trench, dug halfway between two isolated Yucca plants 66 feet apart, 
revealed the presence of laterals. Thus it may be seen that these large plants 
absorb water and soil solutes over a very extensive area in direct competition 
with the grasses. 

A trench 6 feet long and 7 feet deep revealed very few Yucca roots below 2 
feet. However, some do occur and penetrate to maximum depths of about 
7 feet. Like the shallower laterals, the deeper ones are poorly branched, even 
the tips having few branches and ending abruptly. The ends are not more 
than 0.25 to 0.2 mm. in diameter. At a depth of 4 or 5 feet, a few roots were 
foimd running horizontally, simulating those of the shallower soils. 



52 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Another plant had a caudex 3 inches in diameter which reached a depth of 
23 inches. Here it was still 2 inches thick. It branched into two equal 
laterals. These were much flattened and distorted, one being less than an 
inch thick but 2 inches wide. These branched and rebranched, as showoi in 
plate 18, c, all ending within a horizontal distance of 16 inches from the base 
of the plant. Each was abruptly tipped with a bud. These deeper branches 
were not so well provided with roots as was the upper part of the stem. 

Agropyrum glaucum. — ^Wheat grass, if at all abundant, is an indicator of 
favorable deep soil-moisture conditions. However, in thin stands and as 
dwarfed individuals it occurs in rather dry places. The plants here described 
were growing in low ground near a ravine. 

The tufts of these coarse plants are connected by stout, tough rhizomes 
about 2 mm. in diameter and from a few inches to more than 18 inches in 
length. From the base of these clumps and from the rhizomes, which lie at a 
depth of about 1 inch, arise mmaerous short horizontal roots. These are pro- 
fusely branched and rebranched to the third and fourth orders, the ultimate 
branches being almost microscopic in size, and thus furnishing a splendid 
surface absorbing system. 

An abundance of coarse, tough roots, from 1.5 to 2 mm. in diameter, pene- 
trated in a more or less vertical direction and others at an obUque angle to a 
maximum depth of over 7 feet. These coarse roots are covered with a brown 
papery cortex, rather readily removed from the large wire-hke nearly white 
stele. These roots are profusely branched with laterals from a few milli- 
meters to 2 or 3 inches long, many of these branches running off more or less 
horizontally. At a depth of about 4 feet many of these roots become only 0.2 
mm. in diameter, but the branches (which are somewhat shorter) are no less 
abundant to the very tip. The roots have such an abundance of root hairs 
that the whole surface appears to be covered with wool. Thus Agropyrum 
is suppHed with an absorbing system which thoroughly permeates all portions 
of the soil to a depth of 6 feet. 

Carex pennsylvanica. — ^This sedge has a wide range throughout the grass- 
land formation. Because of its early growth and flowering habit, it forms 
conspicuous societies in the prevernal aspect, often before the taller gi*asses 
resume growth. 

The roots of a number of plants of this species were examined. The tufts 
are connected by coarse rhizomes, 2 to 10 inches long, at a depth of 1 to 3 
inches. The much-branched flbrous roots have a lateral spread from the base 
of a tuft of only 2 to 3 inches. They originate from the rhizomes as well as 
from the base of the clumps. While many of the roots, after sending off 
abundant laterals which branch to the third and fourth order, end at a depth 
of 12 to 14 inches in exceedingly well-branched tips; others penetrate deeper. 
A few roots were traced to their delicate endings at a depth of 3 feet, while 
others occurred below the second foot. The chief absorbing area lies within 
the first and second feet of soil. 

Andropogon scoparius. — The httle bluestem occurs throughout the grass- 
land formation wherever enough water is present to support its growth. In 
the drier plains association it is often confined to slopes mth a rough topog- 
raphy and greater water penetration or to sandy soil. It dominates the short- 
grasses under the latter soil condition and forms an extensive bunchgrass 
consocies. Under the still more favorable moisture-supply of the prairies it 
abandons the bunch habit and becomes a sod-former. The lateral spread of 
the roots is about 1 foot on each side of the plant and within 4 inches of the 
surface. The roots are very abundant up to the very surface, thus affording a 



THE PLAINS ASSOCIATION. 



53 



distinctly shallow absorbing system. Many of the laterals run out to a dis- 
tance of 12 to 14 inches and at a depth of only 1 to 4 inches, while below and 
inside of these at all angles to the vertical they are very abundant. The 
maximum depth for several roots was 6 feet. They are very abundant to a 
depth of 3.0 feet. The surface absorbing roots were especially well branched. 

Stipa comata. — This grass often appears to be a dominant in the plains 
association where overgrazing has not thrown the advantage in favor of the 
short grasses. Although its aerial part is not extensive, it has a much deeper 
and better developed root sj^stem than the corresponding species of the 
prairies, >S. spartea. A trench 6 feet long and 5.5 feet deep was dug in a nearly 
pure Stipa community. From the base of the clumps arise exceedingly 
numerous fibrous roots, 1 mm. or less in diameter. While many of these 
descend vertically, others run off at various oblique angles, some of them 
having a lateral spread of more than 18 inches from the base of the plant, 
while at a distance of 14 inches horizontally they reach a depth of 6 inches 
(plate 19, a). Beginning at the very surface of the soil, the main roots are 
clothed with relative!}^ short but well-branched laterals, 2 to 20 mm. long. 
Thus the fii-st 28 inches of soil are thoroughly occupied by the main roots, 
which are only a few millimeters apart, the interstices being completely 
occupied by horizontal branches. Even to a depth of 32 inches the soil is 
fairly well filled with much kinked and rebranched threadlike rootlets, while 
not a few reach a maximum depth of over 5 feet. Thus Stipa comata is pro- 
vided with a much finer, more branched, more widely spreading, and deeper 
root system than is Stipa spartea of the prairies. 

Lithospermum linearifolium. — This is a common and rather conspicuous 
plant of the grassland formation. Eastward it forms vernal societies. The 
plant examined had a crow^n of 10 stems arising from the top of the tap-root, 
which was 13 mm. in diameter. The tap proceeded straight downward 2.5 
inches, where it divided into two equal branches, each about 7 mm. in diam- 
eter. These branches then took a downward course, their diameter decreasing 
very rapidly, so that at a distance of 22 inches from the surface they were not 
more than 1.5 mm. wide. This diameter remained almost uniform for several 
feet. They continued their downward couree almost vertically, one part 
branching again at a distance of 3.5 feet and the other at a depth of 5 feet. 
These branches of the second order also took a downward course, being not 
more than 0.8 mm. in diameter. Lower down, one of these branches again 
branched and penetrated to the maximum depth of 10 feet, ending in hairlike 
termini. The root was very dark, almost black in color, and had a papery 
cortex which could be removed very easily, revealiag a white hard and very 
brittle stele. Throughout the lower 5.5 feet the roots were fairly well supplied 
with short, attenuated branches 0.5 to 3 cm. long. Above this point shorter 
branches were few in number. Yevy Httle absorption took place in the first 

4 feet of soil (fig. 14). 

Lygodesmia juncea. — Although this plant is of frequent occurrence, espe- 
cially in drier situations, it plays a role of no great importance in the grass- 
land. It is interesting because of its xeroid-shoot habit. Two plants were 
examined. Each had a tap-root about 7 mm. in diameter, which took an 
almost vertically downward course, in one plant penetrating to a depth of 

5 feet 8 inches. At 10 to 12 inches from the surface strong laterals were given 
off, which (after taking a horizontal course from 8 to 22 inches) turned down- 
ward not unhke the tap-root. Practically no small branches were given off 
and only very seldom, as shown in figure 15, did any branching at all occur. 
The roots throughout, both tap and laterals, pursued a very zigzag and tortu- 



54 



THE ECOLOGICAL RELATIONS OF ROOTS. 




Fig. 13.— Argemone platyceras. Fig. U.—LUhospermum 

linearifolium. 



THE PLAINS ASSOCIATION. 



55 



ous course. The roots were dark brown with a very fleshy cortex, extremely 
brittle, and when broken exuded a milky latex. At the top the diameter was 
about 5 mm. As the root proceeded downward the diameter fluctuated from 
4 to 7 mm., but gradually grew smaller to the tips, which were about 1 mm. 
in diameter and ended abruptly. These roots are very different from the 
much longer but almost unbranched roots of the same species growing in 
loess soil (p. 16). 

Aragallus lambertil. — This loco unfortunately is a widely distributed legume 
throughout the plains grassland, forming one of the most important of the 
vernal societies. As is characteristic of most legumes, Aragallus has a strong 
tap-root. At the top this was 9 mm. in diameter and with a long gradual 
taper it penetrated the soil with a tortuous course almost vertically downward 
to a maximum depth of 8 feet. For the first 4 inches of its course there were 
scarcely any branches except 3 or 4 small hairhke laterals, which penetrated 
horizontally for a distance of 2 or 3 inches. Below this point and to a depth 
of 30 inches the tap was marked by rather profuse, small laterals, ranging 
from hairhke to 0.7 mm. in diameter. These penetrated the earth in a more 
or less horizontal direction for a distance of 2 to 8 inches; they were them- 
selves branched to the second or third order. Below 30 inches the tap was 
veiy sparingly branched throughout its course, these branches being of the 
same character but much smaller than those already described. The tap 
ended by dividing into 3 or 4 ultimate rootlets, 3 or 4 inches long. The root 
was tough, rather woody, light brown in color. Three plants were excavated 
and examined (fig. 16). 

Petalostemon purpureus. — This and the following subdominant have a 
wide distribution throughout the grassland formation. Their presence in the 
plains association is indicative of at least fairly good soil-moisture conditions. 
They reach their best development in the prairies, where they form typical 
societies. 

More than a dozen plants were examined. They have tap-roots varying 
from 3 or 4 mm. to 1 cm. in diameter. The general root system of all was 
very similar. At a depth of 1 to 4 inches they invariably throw off 3 to 7 
large laterals in a rather horizontal direction. These run off in the shallow 
soil almost parallel with the surface and often not more than an inch below 
it, to a distance of 8 to 18 inches before turning rather abruptly downward. 
Usually 3 or 4 of these laterals are more pronounced than the others, some of 
them being as large as 6 mm. in diameter. Like the tap-root, they taper very 
rapidly and thi'oughout their horizontal course give off both large and small 
sublaterals. These are frequently minutely branched and compete with the 
grasses for the water in the surface soil. Upon turning downward, these 
laterals, now usually not more than 1 to 2 mm. in diameter, pursue a course 
in general vertically downward, though more or less curved backward and 
forward. The tap-root descends vertically or runs off 6 to 12 inches from 
this direction hke the laterals, and tapers so rapidly that at a depth of 1 foot 
it is seldom more than 1 or 2 mm. in diameter. Below this level, all the roots 
are more or less threadlike, branching profusely, especially with small 
rebranched feeders varying in length from a few millimeters to several inches. 
The depth of the lateral branches usually does not exceed 4 or 5 feet, although 
some were traced to a depth of more than 6 feet. The maximum depth of a 
tap-root 6 mm. in diameter was 5.5 feet, while another 9 mm. in diameter 
reached a depth of 6.5 feet. Unlike the white prairie-clover, this plant is well 
adapted to absorb the moisture in the surface soil, but hke the former the roots 
occur quite abundantly to a depth of 5 or 6 feet, through a cross-section of 3 or 



56 



THE ECOLOGICAL RELATIONS OF ROOTS. 




Fia. 15. — Lygodesmia juncea. Fig. 16. — Aragallus lamhertii. 



THE PLAINS ASSOCIATION. 



57 



4 feet square under the plant. They are orange-brown in color, the deep roots 
especially being easily followed and distinguished from their competitors by 
this character (fig. 17). 

Petalostemon candidus. — A group of more than 8 stems, all in blossom, 
arose from a woody tap-root 17 mm. in diameter. At a depth of 3 inches this 
tap spht into thi'ee strong parts, 5, 6, and 8 mm. in diameter, respectively. A 
larger branch ran obliquely for a distance of 3 inches and divided into two 
equal parts, each 3 mm. in diameter. One of these descended rather verti- 
cally, but with long, loose loops, 2 to 4 inches in width, turning back and 
forth as was characteristic of other roots examined. The second fork ran off 
obHquely about 4 inches further before turning downward. At a depth of 12 
to 18 inches both branches divided and rebranched again and again, so that 
at the 18-inch level none of the roots were more than 1.5 nam. in diameter. 
Below the second foot the roots ran for long distances, following much curved 
and tortuous courses but vnth. little change in diameter. The branches that 
were given off at frequent intervals were very long and not much branched. 
Finally, in the soil from the third to the fifth foot the branches became more 
numerous and almost microscopic, the root-tips being well suppUed with 
laterals only a few millimeters in length. 

The other two main branches were very similar to those described, and 
spread in such a manner that a cross-section of the absorbing area under the 
plant would not include more than 4 square feet. Several of the hairUke 
ultimate rootlets reached a depth of over 5 feet; the deepest one examined, 
that of the vertically descending tap, reached a maximum depth of over 5.5 
feet. After examining about a dozen species of each color, it was found that 
the purple prairie-clover had many more superficial branches, w^hich lie nearer 
the surface. The roots are yellower in color, the older ones being almost 
black. The branches are fewer, seldom more than 3, and run downward at a 
much sharper angle. The types described are very characteristic. Thus it 
may be seen that the white prairie-clover, Petalostemon candidus, in com- 
parison with the purple praii'ie-clover, Petalostemon purpureus, is not suppHed 
with absorbing roots in the surface 18 inches of soil, but gets the bulk of its 
water and nutrients below this depth. The chief difficulty encountered in 
excavating these plants was that of following the minute, dark-colored 
terooini in the third to the sixth foot of soil (plate 19, b). 

Eriogonuin jamesii. — This species is widely distributed over the plains, 
where it forms summer societies. The thick woody root, 3 cm. in diameter, 
gave rise to a large number of prostrate stems which formed a mat about 8 
^ inches square. Within the first 6 inches of soil, 10 or 12 laterals, the largest 
of which was 5 mm. in diameter, ran off horizontally for a distance varying 
from a few inches to 2 or 3 feet before turning downward. The tap tapered 
gradually, so that at a depth of about 3 feet it was still 5 mm. in diameter. 
To this depth it also gave off 2 other strong laterals and numerous smaller 
ones, as in the sm'face 6 inches of soil. The former pursued a sinuous course, 
such as is characteristic of Eriogonum roots, to a depth of several feet. At a 
depth of about 3 feet, 2 other laterals arose which were only slightly smaller 
than the tap. With the tap and rarely more than 12 to 18 inches from it, 
these pursued an irregularly downward course, often turning backward and 
forward horizontally or almost so, through a distance of 6 to 8 inches or more. 
In the main, however, these roots as well as the tap had a vertically descending 
direction. They were very poorly branched, rarely giving off small unbranched 
wireKke laterals. The roots were traced to a depth of 7 feet 3 inches, where 
they were stiU 2 or 3 mm. in diameter, respectively, and they undoubtedly 



THE ECOLOGICAL RELATIONS OF ROOTS. 




Fig. 17. — Petalostemon purpureus. 



THE PLAINS ASSOCIATION. 



59 




Fig. 18. — Ratibida columimris. 



penetrated several feet further. In color the root is reddish brown and covered 
with a scaly, shreddy cortex, the older parts being quite woody (plate 19, c). 

Ratibida columnaris. — This species is widely distributed throughout the 
grassland formation, where it forms summer societies dominating large areas. 
It has a strong tap-root, 8 to 10 mm. in diameter. For the first 5 inches it 
tapers very rapidly until at the end of 
that distance it is not more than 3 mm. 
in diameter. From that point it passes 
vertically downward \Ndth a zigzag, 
irregular course, very crooked and 
crinkled, tapering slightly (at 12 inches 
being 1.5 mm. in diameter) to a max- 
imum depth of 2 feet. The first 4 or 5 
inches is marked by an extraordinary 
number of more or less horizontal 
branches which vary in diameter from 
0.2 to 0.5 mm.; these extend out 
through the surface soil to a distance 
of 6 to 12 inches and are themselves 
profusely branched with small hairlike 
rootlets from 0.5 to 2 cm. long. The 
number of these primary horizontal 
branches is very large; one count 
showed approximately 13, while 
another showed about 40. Below 4 or 

5 inches the root is seldom branched, 
what branches there are being hairlike and from 1 to 3 cm. long. Near the 
end, however, the tap usually branches into two or three small, short rami- 
fying parts. The root is dark brown in color, rather tough and unjdelding. 
Four plants were examined (fig. 18). 

Senecio aureus oblanceolatus. — Senedo is a very common and often abun- 
dant plains species, frequently forming vernal societies in the gra^'sland. The 
specimens examined were small plants 6 to 10 inches high. The multiple roots 
are so finely di\ided as to be almost fibrous. The number originating from 
the crown varies from 15 or less to 30 or more. Some of these roots take a 
downward course from the crown, but many of them pass out at a sHght angle 
from the surface to a distance of 4 to 12 inches. The point from which they 
start varies from a few milhmeters to 2 inches below the surface of the ground, 
and the point at which they turn doTvuward varies from 2 to 8 inches. These 
roots then penetrate to a maximum depth of over 3 feet. The diameter of 
these roots at the starting point varies from 0.2 to 1 mm. Throughout the 
surface soil they are branched and rebranched to the third and fourth orders, 
and each group of branches varies from 1 mm. to 3 inches in length. Below 

6 inches these main roots, after starting downward, taper until they are not 
more than 0.2 mm. in diameter. They are more or less branched, the branches 
being almost hairhke in diameter and 3 to 10 mm. long. In color, the roots 
vary from a very light brown to a pure white. They are extremely brittle and 
easily broken. Five plants were examined (fig. 19). 

Asclepias verticillata pumila. — This dwarf milkweed is generally distributed 
throughout the hard lands of the plains. It forms extensive midsummer clans. 
The stems vary from 3 to 5 inches in height and are connected by an extensive 
underground system which varies in diameter from 1 to 2 mm. These con- 



60 



THE ECOLOGICAL RELATIONS OF ROOTS. 



necting stems extend from 2 to 5 inches in depth and range in distance between 
the plants from 1 to 15 inches or more. At the points from which the above- 
ground stems take their origin, one or two roots were found which penetrated 
the soil, usually vertically but occasionally obHquely downward, following a 
tortuous course. These vertical roots at the outset may range from 0.5 to 
2 mm. in diameter and gradually taper downward to their extremity, which is 
more or less branched and hairlike. The maximum penetration found was 46 





Fig. 19. — Senecio aureus oblanceolatu^. 

inches. All parts of the root system, beginning about 2 inches below the soil 
and extending to a depth of about 10 inches, are thickly supplied with small 
laterals, usually about 0.3 mm. in diameter and from 0.5 to 3 inches in 
length. These small laterals often come off in groups of 2 or 3. They are 
themselves branched, ending in very minute capillary termini, thus affording 
a very excellent system for surface absorption. The roots are white in color, 
herbaceous, and for roots of this character fairly tough. When cut, small 
amounts of latex exude. Six plants were examined (fig. 20). 



THE PLAINS ASSOCIATION. 



61 



Opuntia camanchica. — This Opuntia enjoys a very wide distribution in the 
plains grassland. Its water requirement seems to be somewhat less than that 
of Bouteloua. It is favored in its competition with the grasses by grazing, and 
its development in great abundance is frequently indicative of overgrazing. 

A single, carefully selected plant of average size was examined. As is 
characteristic of the cacti in general, the root system consists of two distinct 
parts — a few vertically descending anchorage and deep absorptive roots, and 









-J- — 








^ 








V 










^ 

V 







Fig. 20. — Underground parts of Asclepias verticillata pumila. 

a much more extensive and shallow surface absorbing system. An examina- 
tion of figure 22 shows 23 roots which run off in the surface soil, usually at 
a depth of about an inch and seldom deeper than 3 inches, to distances 
varying from 6 inches to 6 feet. The two largest roots were 5 mm. in diameter 
and tapered very slowly in spite of the repeated branching. They were 
branched repeatedly from their origin at the base of the plant to their 
extremity with both large and small branches, which ramified in all directions 
and thus furnished an enormous absorbing surface. The ultimate root end- 
ings, whether of the shorter or larger branches, consisted of much-branched 
and very delicate brushlike termini, to which the soil clung with great tenacity. 
The smaller branches, except for their lesser extent, are sinailar to those already 
described. 

The deep anchorage and absorbing system of this plant consisted of 4 main 
roots, which at the outset were 2.5 to 3 mm. in diameter. These extended 



62 THE ECOLOGICAL RELATIONS OF ROOTS. 

almost vertically downward, following a more or less irregular course through 
the hard soil, branching as shown in figure 21 and reaching a maximum depth 
of 35 inches. While the shallower roots were brownish in color and very- 
tough, these deeper roots were glistening white and much more fragile (cf. 
Preston, 1900). 




Fig. 21. — Opuntia camanchica. Fig. 22. — Top view of surface roots of Opuntia caman- 
showing vertical anchorage chica, showing the wide lateral spread, 

roots. 



Opuntia fragilis. — ^This cactus has a distinctively shallow root system. On 
all sides roots extended out from the base of the plant, ranging from 1 to 2 mm. 
in diameter, some having a lateral spread of 16 inches. There were about 20 
on the plant examined. Most of these took a course parallel with the 
surface of the soil and lay within 3 inches of the surface, but a few took a 



THE PLAINS ASSOCIATION. 



63 



slightly more downward course, penetrating to a depth of not more than 8 
inches. The first 3 or 4 inches of the root were sparingly branched, the few 
branches being mostly mere tufts of rootlets not more than 3 or 4 mm. in 
length. Throughout the rest of their course these divergent roots were 
abundantly supplied with a system of laterals 0.2 to 0.3 mm. in diameter and 
1 to 12 cm. long. These laterals were branched and rebranched to form 
extensive tufts interspersed throughout the soil, the whole forming a most 
excellent system for surface absorption. Another plant gave a root depth of 
15 inches. 

PLAINS ROOT SYSTEMS AND THE PLAINS ENVIRONMENT. 

An examination of the foregoing data impresses one with the great 
depth to which the roots of most plains plants penetrate. There are 
only 3 species of shallow-rooted plants that seldom extend below the 
first 2 feet of soil {Kceleria cristata^ Opuntia fragiliSj and Ratihida 
columnaris) . Perhaps Opuntia camanchica should also be placed here, 
though its vertically descending roots reach depths of 3 feet. This 
group constitutes only 11 per cent of the 28 plains species examined, 
while in the prairies 6 grasses or 18 per cent of the 33 species ex- 
cavated had shallow roots. In the second group, made up of plants 
with roots extending well below the second foot of soil but seldom 
deeper than 5 feet, are found 32 per cent of the plains species as against 
27 per cent among prairie plants. The third group of deep-rooted 
plants, which extend to depths greater than 5 feet, contains 57 per 
cent of the plains plants and 55 per cent of prairie species. 

On the other hand, certain prairie plants, such as species of Rosa, 
Lygodesmia, Liatris, and Kuhnia, reached depths of 17 to 22 feet. This 
was quite beyond the greatest depth (about 13 feet) attained by any 
plains plant. It may be that the soil is never moist to greater depths on 
the plains. Moreover, most of the plains species are characterized, in 
addition to their great depth, by a fine system of surface absorbing 
and wide-spreading laterals. However, we must again resort to an 
examination of the habitat factors in order to properly interpret the 
root development. 

The region in which these plants grow has an annual precipitation 
of only about 15 inches, an amount quite typical for much of the 
Great Plains area. Fortunately, the major portion falls during the 
growing season. There is as a rule considerable rainfall during the 
months from May to September, but often the rainy season covers 
only June, July, and August. The mean monthly and annual precip- 
itation in inches for Colorado Springs, together with its seasonal dis- 
tribution, is shown in table 14. 

Such a seasonal distribution of moisture is very favorable for the 
growth of grasses. Because of the great compactness of the soil, brought 
about in part by the binding influence of the short-grass cover, the run- 
off is very high. Shantz has shown that at Akron, Colorado, the run-off 



64 



THE ECOLOGICAL RELATIONS OF ROOTS. 



varies from 15 to 43 per cent, the greater run-off naturally occurring 
during heavier rains. This combination of a meager amount of 
precipitation with a relatively low degree of penetration would lead 
one to think the soil must be quite dry. Weekly soil-moisture deter- 
minations at various depths for the summer of 1918 are shown in table 
15, which gives the available water-content during the summer of 
1918. The minus sign indicates that no moisture was available for 
plant growth. 

Table 14. 



Time. 



Jan. . 

Feb. 
Mar, 
Apr. 
May 
June 
July, 



Amount. 



0.20 
0.32 
0.75 
1.55 
2.43 
1.88 
2.81 



Time. 



Aug 

Sept 

Oct 

Nov 

Dec 

Annual . . 



Amount. 



2.20 
1.17 

0.67 
0.35 
0.25 



14.58 



An examination of these data shows that, aside from variations in 
the surface foot, the soil to a depth of 7 feet was rather uniformly dry. 
How much more deeply the rain may penetrate during a series of 
wet years was not determined. Certainly, available moisture must 
occur at least locally to depths reached by living roots of Psoralea^ 
Argemone, and other species which penetrate to 10 or 12 feet. Only a 

Table 15. 



Date. 


Depth 
to 6 
inches. 
Wilting 
coeffi- 
cient 7.9. 


Depth 
6 to 12 
inches. 
Wilting 
coeffi- 
cient 8.4. 


Depth 
1 to 2 
feet. 
Wilting 
coeffi- 
cient 4.9. 


Depth 
2 to 3 
feet. 
Wilting 
coeffi- 
cient 5.1. 


Depth 
3 to 4 
feet. 
Wilting 
coeffi- 
cient 6.5. 


Depth 
4 to 5 
feet. 
Wilting 
coeffi- 
cient 6.8. 


Depth 
5 to 6 
feet. 
Wilting 
coeffi- 
cient 8.7. 


Depth 
6 to 7 
feet. 
Wilting 
coeffi- 
cient 7.0. 




0.6 
-1.0 
-0.5 
-3.5 
-2.8 

5.0 
-2.0 
-4.7 
-4.0 
-3.4 


-0.5 
-0.5 
-1.0 
-3.3 
-2.4 
0.8 
0.0 
-3.2 
-4.0 
-3.0 


1.8 
3.5 
3.2 
1.2 

-0.6 
1.1 
1.8 
0.4 

-0.4 


0.3 
3.9 


-0.8 
2.0 








June 14 


0.7 


-0.4 


0.3 


June 26 


July 1 












July 8 












July 15 


2.5 


1.1 


1.6 


1.9 


4.4 


July 29 


Aug. 5 












Aug. 12 


-1.0 


-0.8 


-0.6 


0.5 


1.8 


Aug. 19 

















long series of soil-moisture determinations, coupled with a dynamic 
study of root growth through at least one wet and one dry cycle, can 
answer this question of moisture penetration and its correlation with 
root development. 

In the short-grass community at Akron, Shantz found that for a 
period of several years little or no water was available for plant growth 



THE PLAINS ASSOCIATION. 



65 



below a depth of 18 to 24 inches. Even a rainfall of 2.4 inches in a day 
had no effect upon the soil moisture below 18 inches. He states that 
^'almost the entire root system of short grasses is limited to the sur- 
face 18 inches," while ''deep-rooted plants are not found here for the 
reason that at greater depths the soil contains no water available for 
their use" (1911: 38). The results obtained by the writer in an area 
of short-grass land locally known as ''adobe" do not correspond with 
these findings. Careful examination was made of the root systems of 
Bouteloua gracilis and Muhlenhergia gracillima in pure short-grass land 
about 25 miles southeast of Colorado Springs (plate 20, a). Here 
Bouteloua gracilis was dominant with Opuntiapolyacantha, while Muhlen- 
hergia gracillima was very abundant in matlike areas. Schedonnar dus 
paniculatus, Senecio aureus ohlanceolatus, Erigeron pumilus, Gutierrezia 
sarothrce, and Munroa squarrosa were present, but were not at all 
abundant. In fact, only 15 to 25 per cent of the soil was covered 
with vegetation. Roots of Bouteloua gracilis were found to be very 
abundant to a depth of 40 inches, while several roots were traced to a 
depth of 51 inches. Muhlenhergia gracillima roots were very abun- 
dant to 50inches, and some reached a depth of 56 inches. 

These findings are so different from those of Shantz that further 
work seems necessary for a thorough understanding of plains root 
systems. This also seems to be true for the roots of sandhill species 
(p. 68). Comparisons of the root habits of these plants with those of 
other communities are based upon the data made available by these 
studies, and may need revision when other regions are worked. The 
great variations of many root systems under different edaphic environ- 
ments is clearly shown in the section on poly demies (p. 110). However, 
it is certain that plains plants grow in a soil of low water-content and 
of very hard texture. The soil in the area where these studies were 
made consisted of a light-colored loam intermixed with some sand. 
It is spaded with extreme difficulty and a hand-pick removes it so 
slowly that in digging the trenches a large pick was kept in constant 
use. Therefore, roots were unearthed only with great labor. For 
example, a single specimen of Argemone platyceras occupied the atten- 
tion of two persons for 1.5 days, while it required the services of a 
third to do the sketching as the roots were uncovered. At depths 
varying from 6 to 10 feet a layer of sand occurred. This was in 
good tilth condition and usually moist enough to hold when pressed 
firmly into a lump. 

The well-developed system of wide-spreading laterals, so charac- 
teristic of species of Bouteloua, Gutierrezia, Artemisia, and many others, 
is undoubtedly a response to the moisture in the surface soil resulting 
from summer showers. Such roots would be of small advantage to 
prairie plants in the Pacific Northwest. In the prairies of Nebraska 



66 



THE ECOLOGICAL RELATIONS OF ROOTS. 



they are probably not developed so extensively because of the greater 
available water in the deeper soils. This matter is further discussed 
on pages 79 and 88. 

A continuous record of the soil temperature was obtained at a depth 
of 4 inches among these roots from June 5 until August 5. At this 
time the bulb of the soil thermograph w*as lowered to 8 inches. The 
most interesting fact disclosed was the extreme variation in tempera- 
ture during the 24-hour period. This ranged from 60° or 70° to 90° 
or %° F. or even more. During August, at a depth of 8 inches in the 
soil, the daily fluctuation was usually about 10° F. within the range of 
70° to 85°. The daily fluctuation of air temperature among the plants 
was usually about 35° to 40° F., the air reaching a maximum of 90° 
or 95° F. in the shade in the afternoon and falling to 50° or 60° F. in 
the morning. Table 16 gives the mean monthly temperatures for 
Colorado Springs. 

Table 16. 



Time. 


Temp. 


Time. 


Temp. 


Jan 


28.0° F. 


Aug 


66.9° F. 


Feb ,,, 


29.8 


Sept 


59.3 


Mar 


37.0 


Oct 


48.4 




44.8 




37.5 


May 


54.1 


Dec 


30.6 


June .... 


63.4 






July.... 


67.9 


Aver . . . 


47.3 



The humidity is relatively low. During clear days, excepting early 
in the morning, it is usually less than 50 per cent and it not infre- 
quently drops to 10 or 15 per cent and sometimes lower. Even at 
night, when the air is coldest, it seldom exceeds 80 or 90 per cent, and 
dew rarely forms. There is usually steady wind movement on the 
plains. Records from a standard anemometer placed just above the 
plants at a height of 0.5 meter show an average daily wind velocity of 
120 miles during the growing season of 1918. Likewise daily water- 
losses from non-absorbing atmometers placed at a height of 10 cm. 
above the soil surface are relatively high. From the data given in 
table 17, which shows the average daily evaporation on the plains, 
it may be seen that the plains habitat is xerophytic as regards both 
air and soil environment. 

In order to visualize clearly the actual root position and the con- 
ditions of competition, the accompanying bisect (plate A) was made. 
Similar bisects are also given for other plant communities. The 
method used is the same in all cases. A representative meter-quadrat 
was selected which showed both dominant and subdominant species. 
After charting and photographing, a trench was dug in front of the 
quadrat, care being taken to have the wall corresponding with a side 



I 




Quadrat-biBect showing the root distribution of certain dominant and subdominant plains species: 
A, Arisiida purpurea; B, Boutdoua gracilis; Ar, Aiiemisia frigida; P, PsoraJm ienuiflora; C, 
Ckrysopsis irillosa; Y, Yucca glauca. 



THE PLAINS ASSOCIATION. 



67 



of the quadrat cut smooth and perpendicular. Then by the use of a 
hand-pick the soil was carefully removed from the roots in the trench 
wall along the edge of the quadrat to a horizontal distance of 4 inches. 
While this was under way the roots were carefully measured and 

Table 17. 



Time. 


Evapora- 
tion. 


Time. 


Evapora- 
tion. 


June 7-10 

June 10-17 

June 17-24 

June 24 to July 1 . . . 

July 1-8 

July 8-15 


c.c. 

52.2 
69.4 
35.3 
66.1 
45.2 
26.5 


July 15-22 

July 22-29 

July 29 to Aug. 5... 

Aug. 5-12 

Aug. 12-19 


c.c. 
32.3 
44.3 
64.5 
49.5 
41.0 



drawn in position t^D scale. This gives a picture of the exact root 
distribution in a block of soil a meter long, 4 inches wide, and 7 feet 
deep, in this particular case. Dotted lines indicate the exit or entry 
of roots from or into this soil area respectively. This plains bisect 
shows the complete occupancy of the soil by roots and the intense com- 
petition for moisture and soil solutes that must result. 



68 THE^ECOLOGICAL RELATIONS OF ROOTS. 

V. THE SANDHILLS SUBCLIMAX. 

In order to compare root development more extensively under 
different environmental conditions, work was continued in a sandhill 
area of Colorado adjoining the plains community and located about 
40 miles southeast of Colorado Springs. Since general climatic con- 
ditions are almost identical with those described for the plains, any 
differences in root development may be attributed to edaphic causes. 
An excellent description of the sandhill communities, considered in the 
light of their successional relation, has been given by Pool (1914). 
Shantz (1911) also gives a thorough treatment of Colorado sandhill 
vegetation from the standpoint of its indicator significance in regard 
to crop production. The general character of the sandhills and their 
characteristic vegetation is shown in plates 20, b, and 21, a. 

Redfieldia flexuosa. — This grass is at home in the sandhills. Indeed, it is 
the most abundant and controlling species of blowout pioneers. While it 
may be mixed with other pioneers, it is often the only plant present in such 
situations. The usually sparse and rather small clumps are connected by 
means of very long, coarse, tough rhizomes, which sometimes reach a diameter 
of 4 to 5 mm. but are usually smaller; they are frequently many feet in length 
and may be traced for a distance of 20 to 40 feet on the surface where the sand 
has been blown away from them. Because of the shifting sand, the depth at 
which they occur is variable. Living rhizomes with vertically descending 
branches were found at a depth of 38 inches and they are rather abundant 
between this depth and the surface, some running horizontally, others 
obliquely and sometimes almost vertically. 

From the nodes of these tough rhizomes, which are from less than an inch to 
6 inches apart, whorls of roots arise in addition to sharp-pointed buds. These 
consist of 2 to 10 roots, but are usually 3 to 5. The roots vary greatly in 
length, diameter, and direction of growth, depending upon the age and posi- 
tion of the rhizome. Those near the tip of the sharp-pointed, much elongated 
rhizome may be less than 1 inch in length, very fleshy, 2 to 4 mm. in diameter, 
and practically destitute of branches. Others a foot or two from the tip are 
12 to 20 inches long and are covered with a fine absorbing system of short 
rootlets, except for the 4 to 6 inches of the growing end. The older roots, 
which are well branched to the very tips with much divided laterals 1 to 3 
inches long, reach a maximum depth of 56 inches. They are often 2 to 4 mm. 
in diameter. They not only run rather vertically downward, but also diverge 
at all angles, even to the horizontal. Long, well-branched roots were traced 
to distances of over 3 feet from the base of the plant at depths of 4 to 8 inches. 
The laterals from these frequently ascend vertically upward and end in well- 
branched termini only 2 to 3 inches below the surface. Thus the length, 
position, and abundance of this rather coarse root sj^stem, together with the 
rhizomes, equip Redfieldia flexuosa in an effective manner for life in the 
shifting but moist soil of the sand-dune (fig. 23). 

Calamovllfa longifolia. — The sand-reed is also an efficient sand-binder. 
It is frequently found associated with Redfieldia fiexuosa, Psoralea lanceolata, 
and other pioneers in the shifting sands of the blowout, but normally occurs 
with Andropogon hallii and others somewhat later in the succession. 

It forms a veritable mat of roots and rhizomes to a depth of 34 inches. A 
wide trench 6 feet long and over 5 feet deep was dug on a small dune which was 



THE SANDHILLS SUBCLIMAX. 



69 



well captured by a dense and nearly pure growth of this sand-binder. This 
plant has a great abundanceof tough, wiry, and very much-branched rhizomes, 
2 to 4 mm. in diameter, which form an underground network connecting the 
apparently isolated plants. These rhizomes are thickly covered with long 
scales and tipped with buds about an inch in length with very sharp, hard 
points. This branched network of rhizomes may be formed in the soil at al 



70 



THE ECOLOGICAL RELATIONS OF ROOTS. 



depths from the surface to 3 feet, the deeper ones undoubtedly having been 
buried by wind-blown sand. From these rhizomes originate multitudes of 
tough, wiry roots, 1 to 2 mm. or less in diameter, which penetrate the soil in all 
directions from vertically downward to horizontally. These roots taper 
gradually and many of them reach a maximum depth of 55 to 60 inches. 

From a depth of an inch to their very tips they are abundantly supplied 
with laterals which extend out more or less at right angles to a distance of 1 or 
2 inches or even more. These laterals are well provided with finer branches 
to the third and fourth order. Since these roots arise not only from the surface 
but also from the deeper rhizomes and penetrate to 3 or 4 feet in all directions, 
they form a confused tangle. Where the grasses are at all thick, as shown in 
plate 21, B, the roots completely fill the soil and bind it so thoroughly that a 
vertical face of a trench 6 feet long and nearly as deep held tenaciously with- 
out caving, even after it was somewhat undercut at the base. 

Andropogon hallii. — This tall, coarse grass forms loose, open bunches with 
only a few large stems. It is a dominant along with A. scoparius in the bunch- 
grass subclimax of the sandhill region. Successionally it occurs somewhat 
later than the preceding sandhill grasses. It was excavated on a partially 
captured sand-dune (plate 22, a). It is a very coarse, glaucous grass con- 
nected by an elaborate system of rhizomes 3 to 4 mm. in diameter and lying 
at a depth of 2 to 6 inches. From the rhizomes many roots take their origin, 
some of which spread laterally, others penetrate almost vertically downward, 
while still others take an oblique course downward. The diameter of these 
varies from 1 to 2 mm. The roots running off laterally take a course almost 
parallel with the soil surface or in some instances curve downward and then 
up again to a point near the surface. These laterals lie at a depth of 2 to 16 
inches and the maximum spread found was 3 feet. Of the roots taking a 
vertical course the maximum depth found was 27 inches; most of them, 
however, penetrated to a distance of about 22 inches or less. The oblique 
roots were of about the same length, but since they pass off at an angle the 
depth depends upon the degree of divergence from the vertical. All roots 
throughout their course were copiously branched with mostly short branches 
ranging from 0.5 to 3 inches in length; as many as 8 of these branches were 
counted on a single inch of the root. These small branches were themselves 
abundantly supplied with minute rootlets 1 to 5 mm. in length. Occasionally 
the main roots gave off a branch 4 to 8 inches long, but these were very rare 
and when they did occur were themselves branched in the same manner as 
the main roots. Altogether, this plant has an excellent soil-binding and 
absorbing system throughout the soil area of each bunch. Eight or nine 
plants were examined. 

Muhlenbergia pungens. — This grass is characterized by tufted stems and 
glaucous, narrow, rigid leaves. The culms, which arise from rootstocks, are 
grouped in small tufts or cushions that lie close to the sand. It is a regular 
component of blowout communities and is not infrequently dominant. 

This important sandhill grass was examined on the rim and grassy top of a 
blowout where it was growing very abundantly. Clusters of roots arise from 
the short rootstocks, which are 2 to 6 inches long and 1 to 3 mm. in diameter. 
These vary in number according to the length of the rootstock, from only 4 
or 5 to 25 or more (plate 22, b) ; the largest are only 1 mm. in diameter. While 
some of the wirelike tough roots penetrate rather vertically downward to a 
maximum depth of 33 inches, others run off obliquely at various angles, even 
to almost parallel with the soil surface. The lateral spread varies from 10 
to 25 inches on either side of the plant. Because of the dryness of the surface 



THE SANDHILLS SUBCLIMAX. 



71 



inch of sand, the ghstening white roots are rather destitute of branches, but 
below this level all of the roots are densely covered with multitudes of very 
fine absorbing laterals. Indeed, a single inch of an average root was found to 
have approximately 75 of these hairlike absorbing laterals. While many of 
them are only a few millimeters long, others reach a length of 6 to 10 inches. 
All are profusely and minutely branched, the larger ones to the third and fourth 
order. Thus this wonderfully efficient root system is able to penetrate all 
portions of the sandy substratum and to extract the available water, while 
at the same time it forms a very effective means of preventing sand from 
blowing. 

Sporobolus cryptandrus. — In aerial habit this grass is not unhke Gala- 
movilfa. It ranks as a dominant among sandhill grasses. Three clumps of it 
were examined in two separate trenches on a half-captured blowout. As a 
whole, it is characterized by an extremely fine fibrous root system, the main 
divisions of which arise from a short stocky rhizome 2 to 4 inches in length. 
On a single rhizome 1.5 inches long, 40 of these main roots were counted. As 
they leave the rhizome they pass off in all directions, some vertically, some 
obliquely, and a great many of them almost parallel with the surface, but 
gradually growing deeper as they pursue their course, so that at a maxi- 
mum distance of 18 to 20 inches many were found at 6 to 8 inches below the 
surface. The maximum vertical depth of penetration found was 22 inches. 
From the point where the major divisions of the roots leave the rhizomes, 
they are covered with a dense growth of root-hairs which hold the sand firmly 
and give the roots the appearance of being much greater in diameter than they 
really are. They are actually threadlike, usually less than 0.5 mm. in diameter 
and very tough. These roots are slightlj^, if at all, branched for a distance of 
2 or 3 inches from the rhizome, but from that point on to the very extremity 
they are well supplied with branches from an inch or less to 8 or 10 inches long. 
These branches pass out in all directions from the main root and are them- 
selves exceedingly finely branched and rebranched to the third and fourth 
order, so that the soil for a depth of from 10 to 13 inches is well filled with this 
fine, delicate absorbing system. 

Eriogonum microthecum. — As shown in plate 21, a, this Eriogonum fre- 
quently controls local areas on sandy slopes and occurs rather widely through- 
out the sandy plains area. The plant has a multicipital stem, which at a 
depth of 4 to 8 inches merges into a strong tap-root 0.5 to 1 inch in diameter. 
At a depth of 2 or 3 inches the tap-root begins to branch, and for a depth of 
2.5 to 3 feet sends off an extraordinary absorbing system (fig. 24). Some of 
these branches arise singly, but it is common to find 2 to 4 springing from 
approximately the same point. In diameter they vary from 1 to 5 mm. The 
general tendency of the branches is to pursue an almost horizontal course from 
a distance of a few inches to 2 feet and then turn almost vertically downward, 
penetrating the sand to a depth of 18 inches to 3 feet. A few of these branches, 
however, instead of taking this course, pursued an almost horizontal course to 
their very tips, which were sometimes 3 feet from the tap. These laterals for 
the first few inches of their course as they left the tap-root were very sparsely 
branched, the distance depending upon the size of the root. Beyond that 
point the branches gradually became more numerous and as one approached 
the extremities the number was so great as almost to form a conspicuous 
network in the fight sand. As the main laterals and their larger branches 
approached their lower extremities, the little rootlets were so numerous as to 
form fairly brushy tips. Below 3 feet the branches of the tap were much 
fewer in number and much larger in size. As a rule they seem to occur in 



THE ECOLOGICAL RELATIONS OF ROOTS. 




Fig. 24. — Eriogonum 



microthecum. 




a ten-year-old Artemisia filifolia. 



THE SANDHILLS SUBCLIMAX. 



73 



groups, ranging from 2 to 5 mm. in diameter. The larger branches, after 
diverging somewhat from the main tap, took a downward course similar to that 
of the main root. At 6 feet the tap divided up into a group of 3 or 4 roots 
of almost the same size, which after diverging took the usual downward course 
and at a depth of 10 feet were 2 to 3 nmi. in diameter. At this depth repeated 
caving of the sand made further digging so dangerous that the work was 
abandoned. 

Artemisia filifolia. — In the sandhill mixed association, aside from the two 
grass dominants, Calamovilfa and Andropogon hallii, the sand-sage is the most 
conspicuous plant. This is due both to its shrubby habit and its great abun- 
dance. It is indicative of a light type of soil with considerable moisture pene- 
tration. It drops out in the more compact hard lands of the plains. 

Three plants were examined. They were growing on a stabiHzed area near 
the sand-dune. The largest had a tap-root 1.5 inches in diameter, from which 
arose a large number of stems to a height of 2 feet, forming a bush about 18 
inches in diameter. The strong, vertically descending, woody tap-root 
tapered gradually and uniformly to a depth of 32 inches, at which point it 
broke up into 3 laterals, the largest 0.5 inch in diameter and the other two but 
slightly smaller. Some of these were traced to a depth of 9 feet, where they 
were still 4 nam. in diameter (fig. 25). In the surface 30 inches of soil the tap 
gave off 6 large laterals, from 6 to 13 mm. in diameter, and a very large 
nimiber of smaller ones, some of them but 0.5 mm. thick. These branches 
formed a rather distinct surface absorbing system, and showed a strong 
tendency to run out in a direction parallel with the surface. In fact, some 
of them terminated in soil which was shallower than the depth at which they 
originated. These with their branches and the numerous rootlets from the 
somewhat oblique larger roots formed a dense network confined to the first 
30 inches of soil. This was characteristic of all the plants examined and was 
not unlike the root habit of Eriogonum. Some of these larger branches ran 
off laterally to distances of 2 to 5 feet before turning downward. Although 
they branched somewhat freely at intervals in the fourth to seventh foot of 
soil, they often pursued their tortuous courses for long distances, sending off 
hardly any laterals. 

The relative abundance of the shallower roots as contrasted with the deeper 
ones, together with their lateral spread and the extent of their branching, is 
well shown in figure 25. Unfortunately, because of the loose texture of the 
soil, it was unsafe to trace them to greater depths. The roots are dark brown 
in color, the older portions having a rough, fissured bark. The younger ones, 
and especially those in the deeper soil, were exceedingly brittle. The plant 
here described was 10 years old. The lower branches and divisions of the tap 
were very sparsely branched, the few rootlets being of the same character as 
those nearer the surface. From the description and figure it will be seen that 
while this species has a strong tap-root, it also has a highly developed absorb- 
ing and binding system characteristic of the general group of sandhill plants. 

Tradescantia virginiana. — This spiderwort is rather common in the sand- 
hills. The plants were examined on a partially captured blowout. They had 
a rather fleshy root system. As many as 18 of these fleshy roots, varying from 
1 to 2 nam. in diameter, were foimd arising from the base of a single stem (fig. 
26); some passed down almost vertically to a maximum depth of 19 inches; 
others started down more or less vertically, then with a gradual curve out- 
ward ended from 6 to 8 inches from the vertical; others passed more or less 
obHquely downward from the stem. The greater part of these fleshy roots, 
however, formed a surface absorbing system which spread out in all directions 



74 



THE ECOLOGICAL RELATIONS OF ROOTS. 



from the base of the stem, running more or less parallel with the soil sm'face 
at a depth of 2 to 5 inches and to a maximmn distance of 30 inches. 

All the roots, whether superficial or deeper, were practically free from 
branches for a distance of 4 to 6 inches. From that point on they began to 
branch, the branches increasing in number toward the tips. These branches 
varied from a few millimeters to 3 or 4 cm. in length, and at intervals on these 




Fig. 26. — Tradescaniia virginiana. 



were found short hairlike branches 2 to 5 mm. in length. While the root 
system of this plant was fairly well branched, the branches were in the main 
not nearly so numerous as those of most of the other sandhill perennials. The 
roots were light brown to white in color, fleshy and brittle. Six plants were 
examined. 

Heliotropium convolvulacemn. — This annual frequently forms small fami- 
lies or colonies on the rims of blowouts and on half -captured sandy slopes. The 
plants were examined on August 1. They were quite mature and in full 
bloom. None was over 7 inches high. They are characterized by a small 
tap-root, seldom over 2 or 3 mm. in diameter, which maintains its dominance 
in spite of the abundant laterals and reaches depths of 18 to 25 inches. 
Throughout its rather vertically downward course, after passing through the 
surface inch of dry sand, it continually gives off laterals, both short and long, 
to the very tip. While many of these exceedingly fine branches do not exceed 
a few centimeters in length, others with a diameter of 0.3 to 0.5 mm. extend 
for a distance of 6 to 20 inches, often in a rather horizontal direction with the 
soil surface. As a whole they are poorly supplied with smaller rootlets. Some- 
times these run out obliquely for a distance and then turn downward. A 
single root may have 4 to 6 of these long laterals. They branch at intervals 
into long threadlike branches, which are themselves only moderately well 
supplied with absorbing rootlets. However, the lack of abundant branches, 
such as occur in many grasses and dicotyledons, is offset by the delicacy of the 
root system, all parts of which are efficient absorbers. These herbaceous 
roots are rather tough; they are light tan in color. 

Petalostemon villosus. — This low, finely branched plant with its dense cover 
of silvery leaflets stands out as a prominent object in the sandhill flora, where 
it regularly plays the role of a pioneer in the blowouts. Sometimes it occupies 
such situations to the complete exclusion of other species. Of the half-dozen 
mature plants examined, all had strong tap-roots, about 1 cm. in diameter, a 
much-branched root system, and a root penetration of 4 or 5 feet. Figure 27 
is illustrative of these. An examination of this figure reveals the numerous 



THE SANDHILLS SUBCLIMAX. 



75 



surface laterals with their large nodules 1 mm. wide and 2 to 3 mm. long, 
their wide lateral spread, sometimes to a distance of 2.5 feet, as well as the 
abundant system of well-branched absorbing laterals. Laterals of various 
sizes, and often with a spread of more than a foot, come off at all depths to 
near the tip. Characteristically these run off somewhat horizontally and then 
turn down rather abruptly. All of the termini are furnished with a profuse 
network of finely branched rootlets. In color the roots vary from light yellow 
and deep orange to brick-red. In diameter the tap was variable, lower por- 
tions often being of larger size than other parts above. 







. . ^ 








r ^ 




























r 




















: ^ 


r 

^• 










A 





Fig. 27. — Root system of Petalostemon villosus. 



Gilia longiflora. — Three plants of this annual, growing on a well-covered 
sandhill, were examined, all were mature and in full bloom; their miderground 
system started with a strong tap-root, varying in size from 7 to 12 mm. in 
diameter. The tap penetrates almost vertically downward, tapering very 
rapidly until, at a point 6 inches below the surface, it is usually not more than 
3 mm. in diameter. From this depth it tapers more gradually, pursuing a 
kinky and tortuous course downward through the sandy soil to a maximum 
depth of 40 to 50 inches (fig. 28). The most marked characteristic of these 
tap-roots is the remarkable number of both large and small laterals thrown 
off within the first 2 to 10 inches of soil. From 8 to 14 of these are often found, 



76 



THE ECOLOGICAL RELATIONS OF ROOTS. 



varying in diameter from 1 to 3 mm., not including nmnerous smaller ones. 
These larger laterals usually pursue a course almost parallel with the smiace 
for a distance of 5 to 22 inches, and then almost invariably turn abruptly 
downward, reaching depths of 10 to 35 inches. Throughout their course they 
branch freely into both large and small rootlets, the larger branches spreading 
widely and then turning downward after the fashion of the main laterals. 
Throughout the course of the laterals and their larger branches, a striking 




Fig. 28. — Gilia longijlora. 



system of fine absorbing rootlets, branched to the second and third order, 
is found. Below the point where the tap throws off the larger laterals, it is 
much more poorly branched, the branches being relatively much smaller and 
shorter, but well supphed with fine rootlets, as are the laterals above. From 
the figure and description it will be seen that although this is an annual, the 
absorptive system is characteristic of that of most of the species found in the 
sandhills. The roots are glistening white and quite tough for herbaceous plants. 

Euphorbia petaloidea. — Euphorbia is a sandhill pioneer which forms both 
families and colonies. The root system begins with a strong, smooth tap- 
root, varying in diameter according to the age and size of the plant, the 



78 



THE ECOLOGICAL RELATIONS OF ROOTS. 



largest examined being 4 mm. in diameter. This tap penetrates almost 
vertically downward. Beginning at a depth of 2 to 3 inches, it sends off 
multitudes of fine fibrous roots which are much branched and penetrate the 
soil on all sides to a distance from a centimeter or less to 3 or 4 inches. In 
addition to these finer roots, the tap sends off longer laterals which traverse 
the soil in all directions, some taking a course almost parallel with the sur- 
face, while others pass off more obliquely. These vary in length, some being 
10 to 12 inches long. The laterals are much branched and rebranched, and 
together with the finer rootlets form an effective absorbing system. Usually 
at a depth of 5 to 8 inches the tap divides up into such a system of large 
laterals as to almost if not quite lose its identity. These lower laterals vary 
from 0.5 to 1 mm. in diameter and diverge more or less obliquely downward, 
some penetrating to a depth of 26 inches or more. The deeper penetrating 
divisions of the tap branch rather freely and end in much divided termini. 
On the whole, however, these lower branches are more sparsely supplied with 
absorbing rootlets than those nearer the surface. The roots are of a light 
brown to white color, are rather fragile, and hard to trace. Six plants of 
varying sizes were examined. 

Psoralea lanceolata. — This legume sometimes replaces Redfieldia flexuosa 
as the pioneer in the shifting soils of blowouts, but normally it belongs later 
in the succession. Its underground parts eminently fit it to succeed in a 
habitat where the ground-line is constantly lowered by wind erosion or built 
up by the same agency. 

The individual plants, which may be a few inches or several feet apart, are 
connected by a more or less horizontal system of rhizomes, which varies in 
depth from 2 or 3 inches to more than 2 feet. These connecting parts vary 
from a few millimeters to more than a centimeter in diameter and may run 
horizontally for distances of 10 to 30 feet or more. At irregular intervals 
along their course occur erect portions, often a centimeter in diameter and 
frequently forked at the top. These give rise to the individual plants, while at 
other intervals, and not always below the erect stems, strong, rather vertically 
descending roots occur, many of which reach a depth of 8 or 9 feet (fig. 29). 
The surface 2 or 3 feet of the root system is poorly supplied with absorbing 
rootlets. In fact, only a few of the major branches have their origin in this 
layer of soil and the ultimate rootlets of these extend much deeper. The main 
system of branches begins in the fourth foot of soil and extends to the 8 or 
9-foot level. This consists of both large and small wide-spreading and well- 
branched rootlets, the laterals often running out to a distance of 2 feet or more 
from the base of the tap. Large root nodules, 1 to 3 mm. in diameter, were 
observed at a depth of 8 feet. One large tap-root was traced to a depth of 
over 9 feet, where it still maintained a diameter of 4 mm. It was not followed 
further, because of the danger of caving the sandy-walled trench. The roots 
were dark brown in color and rather tough. 

Ipomoea leptophylla. — The bush morning-glory is a common plant in the 
sandhills, where the large hemispherical tops, conspicuous because of their pro- 
fuse purple blossoms, cover many square feet. Two plants which were growing 
together were examined. The top measured 7 feet in diameter and was 30 
inches high. The multicipital stem arose from strong tap-roots 2.5 inches in 
diameter. At a depth of 9 inches the taps became greatly enlarged, reaching 
a diameter of 6 to 8 inches, which they maintained to a distance of about a 
foot, when they tapered off gradually, so that at a depth of 4 feet they were 
but an inch or two in diameter. Below this point the tap lost its dominance, 
breaking up into large numbers of deeply penetrating and widely spreading 
branches, as shown in figure 30. 



Fig, 30. — Ipomaa leplophylia, showiog a portion of the very extensive root system. 



THE SANDHILLS SUBCLIMAX. 



79 



In the first foot of soil scarcely any laterals originated, but below this point 
both large and small branches came off in great abundance. These varied in 
diameter from less than 1 mm. to half an inch. While a very few ran some- 
what vertically or obliquely downward, the general course of most of them 
was quite parallel with the soil surface. The lateral spread was enormous, 
the roots running off to distances of 15 to 25 feet or more. One of the larger 
forks of a root, 0.5 inch in diameter, was traced to a distance of 14 feet and at 
a depth varying from 25 to 30 inches from the base of the plant, when it 
branched into two equal parts, 4 mm. in diameter. These wide-spreading 
laterals sent off abundant branches, many of which passed off at right angles, 
either vertically downward or vertically upward, for distances of many feet. 
Indeed, the surface foot of soil as well as the 10 feet below it were literally 
filled with the glistening white, brittle branches of this remarkable root system. 
The ultimate branches, however, while occurring in great numbers, were not 
profusely divided. The great network of roots originating at a depth of 5 feet 
is well shown in figure 30, but because of the caving of the sandy soil it was 
impossible to follow them to a greater depth. However, judging from their 
diameters of 2 to 4 mm. at this level and the nature of the other roots examined, 
it is highly probable that they penetrated many feet deeper. The enlarged 
portion of the tap-root not only furnishes an enormous reservoir for food, but 
also a storehouse of water upon which the plant may draw during a period of 
drought. 

SANDHILL ROOT SYSTEMS AND THE SANDHILL ENVIRONMENT. 

Eight of the 19 sandhill species examined^ have roots which are 
entirely or nearly confined to the first 2 feet of soil. Of the others, all 
but one have their main root development in the first 2 or 3 feet of 
soil. Even the very deep-rooted Eriogonum microthecum and Arte- 
misia filifolia show a striking profusion of long, wide-spreading laterals 
in this soil layer. Indeed, it will be shown later that Stipa comata, 
Bouteloua gracilis, and Chrysopsis villosa, all rather deep-rooted plains 
species, become rather shallow-rooted under sandhill conditions. 
Among the grasses the rhizome habit is extremely well-developed, 
while long, rather horizontal, shallow laterals are characteristic of 
most of the plants examined. In this soft substratum such a root 
habit can not be attributed to the mechanical difficulty of penetration, 
but seems closely related to the water-supply. 

Rainfall is at once absorbed and there is practically no run-off, even 
in the heaviest showers. As soon as the storm passes, evaporation 
dries out the surface sand with great rapidity, but to a slight depth 
only. This surface layer of dry sand forms an excellent mulch, which 
has a wonderfully retarding effect upon further evaporation. At a 
depth of but a few inches below the surface the sand is always moist 
and may usually be molded into lumps by pressure of the hand. The 
vegetative cover is usually rather sparse and consequently water- 
losses by transpiration should be much less than on the plains. Unfor- 
tunately few data on soil-moisture in sandhill soils are available. 

* Only 14 are here described; the others will be found under the section on ecads (p. 110). 



80 THE ECOLOGICAL RELATIONS OF ROOTS. 

Bates (1910) gives us the following table of water-contents for soils 
taken in the sandhills at Halsey, Nebraska : 



Table 17. — Amount of moisture in the soil, per cent of dry weight. ^ W 





At 1 foot. 


At 2 feet. 


At 3 feet. 


At 6 feet. 


Station. 


























May. 


July. 


Sept. 


May. 


July. 


Sept. 


May. 


July. 


Sept. 


May. 


July. 


Sept. 


South slope 


4.1 


1.9 


3.1 


4.8 


1.6 


3.1 


4.3 


1.8 


3.2 


3.9 


2.8 


3.3 


Bottom 


5.1 


2.9 


4.7 


4.8 


2.1 


4.2 


5.4 


3.3 


4.2 


6.5 


7.4 


8.6 


North slope 


5.1 


3.0 


4.7 


5.3 


3.3 


5.0 


5.9 


5.1 


6.7 


5.9 


7.3 


7.4 




3.3 


4.0 


3.6 


4.4 


3.8 


3.9 


5.2 


3.7 


3.7 


5.4 


4.2 


4.9 



From these data it may be seen that the soil was rather uniformly 
moist to a depth of 6 feet. We must keep in mind, however, that these 
data were taken under a rainfall of about 23 inches and during the wet 
phase of the climatic cycle. 

A single set of duphcate determinations, taken in the area studied 
on July 30, showed the water-content to be rather uniform to a depth 
of 3 feet (about 4 per cent), 3 per cent of which was available for plant 
growth. While further determinations were not made, in all of the 
numerous trenches dug the sand was perceptibly drier at greater 
depths. It seems certain that under the normal light rainfall the 
surface soils from 4 to 36 inches would contain the most moisture, 
while the deeper soils would become wet only during the years of 
abnormal precipitation. The latter may account for the deep root 
habit of Psoralea lanceolata and others. The deeper soils, once wetted, 
would dry out very slowly, because of the relatively small number of 
plants drawing their water-supply from them. Soil nutrients may be 
a Hmiting factor, but this seems improbable in view of the luxuriant 
growth and complete occupancy of the soil by plants when sufficient 
water is supplied. 



THE GRAVEL-SLIDE COMMUNITY. 



81 



VI. THE GRAVEL-SLIDE COMMUNITY. 

A fine series of diverse habitats for investigation are found in the 
mountains adjoining the plains. During August a detailed investiga- 
tion was made of the roots of plants at an altitude of about 8,000 feet 
in the Pike's Peak region of the Rocky Mountains. Here the soil is 
composed of disintegrated granite, the degree of disintegration and 
decomposition determining largely the type of plant community 
occupying any particular area. The successional sequence and species 
belonging to each community of the sere may be found in pubHcations 
by Clements (1904, 1905). The first herbaceous plants to occupy the 
new soils formed by the crumbling granite on the steep mountain slopes 
are members of the gravel-sUde community (plate 23). The semibare 
areas thus populated are so extensive and the fife conditions so unique 
that a rather detailed study was made of the most important species. 

Krynitzkia vu'gata. — ^This striking plant has an erect, spike-like stem seldom 
reaching a height greater than 8 to 12 inches in this habitat. It is a principal 
species of the gravel-shde community. It has a tap-root 4 to 9 mm. in diam- 




FiG. 31. — Krynitzkia virgata. The heavy slanting line indicates the actual ground-line. 



82 



THE ECOLOGICAL RELATIONS OF ROOTS. 



eter, which reaches depths varying from 20 to 30 inches (fig. 31). Except for 
the first 3 to 5 inches, where the plant tops have shpped down the gravel 
sUde, the root pursues a rather vertically downward course, the tip being only a 
few inches horizontally away from the base of the plant, in spite of the back- 
ward and forward meanderings of the root through bends of 3 or 4 inches in 
diameter. The tap is often flattened and kinked where it forces its way 
through the crevices of the semi-decomposed rock. The number of larger 
branches is few, usually not exceeding 2 or 3. One plant gave off only one 
large branch, about 2 man. in diameter, at a depth of 3 inches. As is charac- 
teristic of numerous other plants examined, this lateral branched freely and 
ran off in a direction nearly parallel with the surface for about 15 to 20 
inches, ending in a network of well-branched rootlets. On another plant a 
lateral ran off more than 36 inches from the base of the stem, branching again 
and again. Other smaller branches arise from the tap in great abundance. 
These are only a few centimeters long, but well-branched and densely covered 
with root hairs. The older cortex is black and of a papery texture, peeling off 
readily. 

Paronychia jamesii. — This small, cespitose perennial forms a consocies of 
the community. It has a tap-root about 5 mm. in diameter. Like all other 
plants on the gravel-shde, the top has been pushed down the slope several 




Fig. 32. — Paronychia jamesii. 

inches. The tap breaks up into several larger laterals, usually 3 to 5, and a 
great number of smaller ones. Some of the laterals run off in a horizontal 
direction to a distance of 30 inches or more, at about 2 to 4 inches in depth, 
branching repeatedly, as shown in figures 32 and 33, and being abimdantly 



THE GRAVEL-SLIDE COMMUNITY. 



83 



supplied with minute, absorbing rootlets. Others run off obliquely, while 
still others parallel more or less the attenuated tap-root, and like the tap are 
abimdantly suppHed with both long and short rootlets, the whole forming a 
wonderfully efficient absorbing system. Compared with the transpiring sur- 
face, the root system is very well developed. The surface view in the figure 
shows the roots at a depth of 2 to 2.5 inches. The dotted lines show where 
they turn downward. No roots were found to penetrate to a greater depth 
than 30 inches. 

Aletes acaulis. — This low umbellif- 
erous plant, which is only 6 to 8 inches 
high in fruit, is the dominant of the 
gravel-shde, often being more abundant 
and conspicuous than all other plants 
combined (plate 23, a, b). From the 
large tap-root arises a large number of 
stems, each multicipital in character; 
47 individual stems were counted on 
a single medium-sized plant. Such a 
group forms a formidable obstacle to 
the rock fragments moved by gravity, 
the latter pushing the top of the plant 

6 to 10 inches downward from the top 
of the well-anchored tap-root. Both 
the main stems and the branches show 
marked wrinkles from profound con- 
tractions. At the base of the shoots of 
this perennial, great clusters of dead 
and decaying gray leaf-bases remain 
attached to the plant. 

The tap-root is often 4 cm. or more 
in diameter. It is dark brown in color 
and quite spongy in texture, as was 
noted for several species in this habitat, 
the fleshy roots probably serving for 
water storage. One plant with a tap 4 
cm. in diameter, from which originated 

7 distinct stem-clusters, gave rise in the 
first 3 inches of soil to numerous laterals ranging from 1 cm. to only a few 
millimeters in diameter (plate 24, a, b) . One of these laterals, 3 mm. in 
diameter, ran off at a depth of about 4 inches and in a direction parallel with 
the soil surface to a distance of 4 feet, giving off numerous branches, both 
large and small, each of which, after branching profusely, ended in a network 
of tiny, much-branched laterals. Another surface lateral, 8 mm. in diameter, 
ran up the slope at an average depth of 5 inches to a distance of 4.5 feet. At 
a depth of 6 inches the tap broke up into 3 parts — 7, 8, and 10 mm. in diameter 
respectively. These were very much curved and twisted. They followed 
the crevices of the rocks and none reached a depth greater than 30 inches, 
but spread laterally to a distance of 3 or 4 feet or more from the base of the 
plant. All of the laterals branched profusely and terminated in the moist rock 
crevices in networks of tiny rootlets. The surface 2 to 5 inches of soil is 
especially filled with these absorbing laterals, but they are abundant through- 
out the soil to the tips of the deepest roots. Plate 24, b, shows a fragment of 
these branches. In fact, the soil is Uterally filled with these absorbing rootlets 




Fig. 33. — Surface view of a single root of 
Paronychia jamesii at a depth of 2 to 2.5 
inches. 



84 



THE ECOLOGICAL RELATIONS OF ROOTS. 



to a point several feet on each side of the plant. A single plant thus lays hold 
of the surface soil to a depth of 18 inches or more within a radius of 4 or 5 
feet from its base.- 

Apocynum androsaemifolium. — ^Large areas, especially of the steeper gravel 
slides, are frequently covered by extensive communities in which this species 
is dominant, often forming famiUes (fig. 34). The vertical portions arising 
from the horizontal rootstock are 2 to 5 mm. in diameter. From these|origi- 




FiQ. 34. — Apocynum androscBmifolium, showing rhizomes and dense network of roots. 

nate groups of 2 to 5 erect stems. The rootstocks He at a depth varying from 
6 inches to 2 feet, but usually at about 8 to 16 inches. These run for distances 
of 6 to 10 feet or more, giving rise at irregular intervals to erect portions 
which bear new plants. Sometimes these connected plants are only a few 
inches apart, while at other times the interval between them may be 3 or 4 feet. 

The vertically ascending parts seldom have large branches, but are well 
clothed with abimdant laterals 1 mm. or less in diameter. Thesefbranches 
divide and subdivide into many branch orders, forming brushlike mats which 



THE GRAVEL-SLIDE COMMUNITY. 



85 



run off in all directions to a distance of 6 or 8 inches or more. However, the 
most profound branching arises from the horizontal rootstocks. Like the 
former, these are not large in diameter, but are branched so profusely, extend- 
ing vertically upward as well as downward and laterally, that they com- 
pletely occupy the soil from a depth of 2 inches to a maximum depth of 4 feet. 
Many of these deeper roots originate from strong laterals 1 to 2 mm. in diam- 
eter, which branch off and run in various directions from the rootstocks. 
Figure 34 illustrates the profound absorbing surface characteristic of these 
gravel-slide plants. In color, the larger roots are dark brown, while the finer 
ones vary from tan to almost white. 

Smilacina stellata. — Families of this plant often occur on steep gravel- 
slides, or they are the dominants of a gravel-slide colony. Stout stems, from 

3 to 7 mm. in diameter, arise at intervals of an inch to more than a foot from 
the stout horizontal rhizomes. The latter are about the same diameter as the 
stem and lie usually at a depth of about 4 to 6 inches, although they are some- 
times shallower where the gravel has rolled away and often much deeper 
where the gravel has covered them (plate 24, c) . Some were found at a depth 
of 30 inches. The rhizomes are much branched, frequently at right angles, a 
single rhizome system often connecting a whole family of plants through a 
distance of 7 or 8 feet or more. These rhizomes furnish not only an excellent 
means for propagation, but also serve as storage organs for these herbaceous 
perennials. 

The plants frequently grow in such dense clumps that the soil at a depth of 

4 to 6 inches is quite filled with dense masses of these rhizomes. Although the 
vertical stems do not give rise to rootlets, the rhizomes are uniformly covered 
on all sides with rootlets about 0.5 to 1.0 mm. in diameter. These run off in all 
directions, including the vertical to a distance of 3 to 6 inches. Although 
they are entirely unbranched, they are completely and uniformly covered with 
a dense coat of root-hairs. Thus the plant is well provided for absorbing the 
moisture in the shallower soil. In addition to these shorter rootlets, groups 
of 2 to 5 larger branch roots arise at frequent intervals at the base of the 
vertical stems or where these stems have been, a place now marked by a seal- 
like scar. These vary from 2 to 4 mm. in diameter and throughout their 
course are densely covered with root-hairs. They penetrate the soil to a 
maximum depth of 44 inches. Through the first 6 to 12 inches of their course 
they are unbranched or at least poorly branched, but from this point they 
branch profusely to the third or fourth order, the branches spreading widely 
and the ultimate rootlets being rather coarse, often 0.2 to 0.5 mm. in diameter. 
Since these branches run obliquely and even horizontally as well as vertically 
downward, they furnish with the shorter roots already described an excellent 
absorbing system. Altogether the finer rootlets are very coarse and poorly 
branched when compared with Aletes, Thlaspi, and certain other gravel-slide 
plants. The entire underground parts vary from tan to white in color. 

Pachylophus csespitosus. — This is a common plant on bare gravel-slides 
where it forms pioneer consocies. The social habit is due to the method of 
vegetative propagation (fig. 35). The plants are frequently connected by 
rhizomes from which arise roots 2 to 5 mm. in diameter, none of which were 
found to reach a depth greater than 45 inches. Frequently at 4 to 8 inches 
deep, they turn off abruptly up or down the slope and run parallel with the 
surface for long distances. Large branches arise from these roots and do much 
of the absorbing for the plant. Of numerous specimens examined, the one 
illustrated in figure 35 is typical. 



86 THE ECOLOGICAL RELATIONS OF ROOTS. 

Both large and small rootlets may arise from the rhizomes. The main 
laterals are well supphed with rootlets and the soil is quite filled with them 
to a depth of 6 to 36 inches. As compared with Aletes and Apocynum, the 
rootlets are coarse and rather poorly branched. They vary from tan to nearly 
white in color. The tap, as in many gravel-slide plants, is rather fleshy. 




Fig. 35. — Pachylophus coespitosiLs. 



Thlaspi alpestre. — This penny-cress is abimdant but rather inconspicuous, 
being only 5 to 8 cm. high. Its tap-root is usually not over 2 mm. in diameter, 
and penetrates the dry surface gravel to a depth of 1.5 to 2 inches before it 
gives off branches. Beginning at this depth, tiny laterals, ranging from almost 
microscopic in size to 1 mm. in diameter, develop in abundance (plate 24, d). 
The larger laterals are relatively few, usually not more than 2 to 5, but the 
smaller ones, many about the size of a fine silk thread or smaller, occur in 
great abundance. Often they run parallel with the surface throughout the soil, 
branching repeatedly in such a manner as to form a delicate mass of rootlets, 
the ultimate termini being minute. The lateral extent is usually not greater 
than 4 to 7 inches and the depth of the plant seldom exceeds 15 inches. To 
really appreciate the deUcacy of the profusely branched fragile root system, 
it is necessary for one to carefully pick away the coarse rock particles. 



THE GRAVEL-SLIDE COMMUNITY. 



87 



Mentzelia multiflora. — Size, duration, and abundance combine to make 
this species a dominant on many gravel-slides. The plants are usually about 
10 to 14 inches high when in full bloom. The roots start with a tap 12 mm. or 
less in diameter, which, after penetrating to a depth of 2 to 5 inches, sends off 
strong laterals as large as 3 or 4 mm. in diameter. These are distinctly shallow, 
run almost parallel with the surface, and are repeatedly branched, the 
fine rootlets ending in hairlike extremities. Although they seldom run more 
than 18 inches from the base of the tap, they are frequently much longer 
because of their curved and twisted course. The tap-root takes an almost 
vertically downward course, except where it follows for a time the crevices in 
the rocky soil, and penetrates to a maximum depth of about 22 inches. 
Although the tap is not so profusely branched as are the shallower laterals, the 
branches of both are very similar. The smaller branches of both the laterals 
and the tap are profusely covered with a fine growth of root-hairs. Although 
this plant, when compared with most others, has an exceedingly well-developed 
absorbing system, it does not compare favorably with the wonderfully exten- 
sive and excellently developed root system of Aletes. However, both are well 
adapted to get the water falling during the frequent mountain showers and 
thrive under conditions where most plants could not grow. The roots are 
almost white in color and very spongy in texture. 

Eriogonum flavum. — ^This plant is a subdominant in the gravel-slide com- 
munity, its masses of yellow flowers making it very conspicuous in the 
autiminal aspect. The specimen had a strong woody tap-root 2 cm. in 
diameter which tapered within a length of 6 inches to only 3 mm. in diameter. 
Here it gave off 2 laterals each 2 mm. in width. These ran off laterally for 
about 2 feet at a depth of 3 to 7 inches. An enormous number of smaller 
profusely branched laterals arose, forming a dense absorbing network about 




Fig. 36. — Eriogonum flavum. 



88 



THE ECOLOGICAL RELATIONS OF ROOTS. 



the plant for a radius of more than 18 inches. These were extraordinarily 
well-branched and rebranched and were very dense. At 5 inches in depth 
another large lateral occurred, and the root broke into 2 nearly equal parts at 
6 inches in depth. One of these ran off into the bank and slightly upward to 
a distance of 2 feet from the base of the crown, but it was really much longer. 
It ended 3 inches below the soil surface. This lateral, scarcely more than 1 
nam. in average diameter, gave off both short and long branches in a profuse 
manner, all being repeatedly branched into minute termini and densely covered 
with root-hairs. They also dipped upward to within 2 inches of the surface 
and downward to 8 or more inches, but were most abundant at 4 or 5 inches in 
depth. The other root at the fork ran out into the slope for over 5 feet at an 
average depth of 16 inches. The roots often run long distances without much 
branching, but the last 2 or 3.5 feet form a great network of branches. The 
roots are reddish in color and relatively tough. None of those examined were 
deeper than 2 feet (fig. 36). 

In another specimen the root system began to divide almost from the 
crown, some of the main parts, however, being much larger than the others, 
varying from 2 to 13 rom. All the rootlets, whether large or small, were 
marked by a diffuse dichotomous branching. Most of the branches were 
superficial, extending to a depth of 1 to 12 inches. These branches were 
themselves branched many times, and the termini of the branches of all sorts 
were persistently marked by capillary roots, ramifying and forming a con- 
spicuous network in the soil. The course of most of these laterals was par- 
allel with the surface. The maximum spread of the main absorbing branches 
was found to be 4.5 feet. A very few of the larger branches take a more 
marked downward course, branching very much the same as the horizontal 
laterals, but the branches are not so numerous nor so long. The maximum 
depth recorded was 39 inches and the tip of the root had wandered 43 inches 
from the vertical. 

GRAVEL-SLIDE ROOT SYSTEMS AND GRAVEL-SLIDE ENVIRONMENT. 

All of the plants of this community are similar in possessing roots 
adapted to secure moisture and nutrients from the surface soil. They 
are characterized by a shallow, widely spreading root system confined 
largely to the first 18 inches of the soil and in fact usually best developed 
in the surface foot. The lack of depth is compensated by a remarkably 
wide lateral extent combined with a profound system of branching. 
An explanation of these adaptations is readily found in a study of 
the soil and the distribution of the rainfall. 

The gravel-slide soils consist of a superficial layer of coarse angular 
rock fragments varying in size from over an inch to a few millimeters 
in diameter. Except during rains this surface layer is very dry. The 
slope is so steep that there is often a constant movement of the rock 
particles down the mountain-side, the course of these moving pebbles 
being marked by depressions looking not unlike the tracks of harrow 
teeth. Most of the plant tops have slipped down the slope from 2 to 
8 or more inches. This surface layer is very efficient in preventing 
run-off, as well as in forming a dry mulch and thus protecting the 
underlying soil from high evaporation. A concrete illustration of the 



THE GRAVEL-SLIDE COMMUNITY. 



89 



effectiveness of this layer is shown in the following experiment. Two 
metal cylinders, 8 inches high and 5.5 inches in diameter, closed at one 
end, were filled with wet soil and one of them was covered with a layer 
of these coarse gravel particles to a depth of an inch. After an equal 
exposure to evaporation for a period of 30 hours, it was found that 
the one container had lost 202 grams of water, which was more than 8 
times as much (24 grams) as evaporated from the soil covered with 
the gravel mulch. In nature, finer particles occupy the interstices be- 
tween larger ones, and hence the gravel mulch must be much more 
efficient. 

Below this surface gravel are about 4 inches or more of fairly well 
decomposed rock, a mixture of coarse gravel and sand. On older 



Table 18. — Water-content of the soil of the gravel-slide and half-gravel-slide during 1918. 







Depth to 6 inches. 


Depth 6 to 12 inches. 


Depth 12 to 18 inches. 




Date. 


Gravel- 


Half-gravel- 


Gravel- 


Half-gravel- 


Gravel- 


Half-gravel- 






slide. 


slide. 


slide. 


slide. 


slide. 


slide. 


June 


8 


4.7 


3.0 


5.1 


3.3 


4.2 


2.8 


June 


11 




3.8 




2.7 




2.9 


June 


24 


5.2 


9.2 


4.3 


4.4 


5.0 


3.2 


July 


1 


3.1 


5.5 


4.0 


4.7 


8.5 


4.2 


July 


8 


3.3 


3.5 


4.2 


2.4 


3.2 


2.2 


July 


15 


4.0 


9.5 


4.5 


7.5 


3.5 


4.0 


July 


22 


6.0 


10.1 


3.9 


5.7 


4.1 


4.3 


July 


29 


4.2 


6.5 


4.1 


3.9 


3.2 


3.2 


Aug. 


5 


4.9 


3.1 


3.2 


2.1 


2.7 


2.1 


Aug. 


12 


4.5 


8.9 


3.0 


4.4 


2.6 


4.3 


Aug. 


19 


5.1 


6.6 


4.3 


3.8 


4.6 


3.4 



sHdes this layer extends much deeper. Below this the soil changes 
from a dark brown to a more reddish color and consists of fairly well 
decomposed granite, which becomes less broken up as one goes deeper. 
At 2 to 4 feet in depth it changes into almost solid rock. The roots 
show a marked tendency to follow the cleavage planes of the rock. 
Below 4 to 6 inches the soil is remarkably compact, and it is necessary 
to remove it with a pick, this sometimes being accomplished with con- 
siderable difficulty. 

An examination of the weekly soil-moisture determinations in table 18 
shows that while the water-content is at no time high, it is rather uni- 
formly distributed throughout the first 18 inches of soil. Owing to the 
extreme irregularity in degree of fragmentation of the rock particles 
and to the heterogeneous nature of these soils, moisture-equivalent 
determinations are not given. The amount of non-available water 
in the gravel-shde soils was found to vary from 1.9 to 7.8 per cent; 
in the half -gravel-slide from 2.0 to 8.6 per cent; and in the soils of the 
forest floor from 2.7 to 10.5 per cent (page 109). 

The rather high evaporating power of the air as measured by non- 
absorbing atmometers is given in table 19. 



90 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Table 19. — Average daily evaporation on the gravel-slide and half-gravel-slide during 1918. 



Date. 



June 6—10 

June 10-17 

June 17-24 

June 24 to July 1 

July 1-8 

July 8-15 



Gravel- 
slide. 



c.c. 
26.6 
51.6 
25.8 
48.2 
35.8 
17.6 



Half- 

gravel- 

slide. 



c.c. 
20.2 
40.1 
22.8 
30.0 
25.0 
13.2 



Date. 



July 15-22 

July 22-29 

July 29 to Aug. 5 

Aug. 5-12 

Aug, 12-19 

Aug. 19-23 



Gravel- 
sUde. 



c.c. 
19.8 
31.5 
49.4 
22.9 
24.2 
54.2 



Half- 

gravel- 

slide. 



c.c. 
15.5 
21.4 
31.1 
17.0 
16.9 
32.0 



One factor greatly accelerating evaporation is the marked wind 
movement. From June 6 to August 23 the average daily wind velocity 
at a height of 0.5 m. was 103 miles. Notwithstanding the high evapora- 
tion losses, nearly all of which occur during the day, the upper layer 
of the soil containing the roots is kept moist by frequent rains. Seven- 




FiG. 37. — Quadrat-bisect showing root relations of gi-avel-slide plants. This was made 
along the front of the quadrat shown in plate 23, b. P, Paronychia jamesii;A, Aletes 
acaulis; K, Krynitzkia virgata. 

teen showers, varying in intensity from 0.1 to 1.85 inches, occurred 
from June 21 to August 28, 1918, while the total rainfall during the 
period was over 10 inches. As already pointed out, none of this water 
runs off except during the heaviest rains, while the loose gravel inhibits 
high evaporation losses. Moreover, the plants are so sparsely spaced 
that only relatively small amounts of water are removed by transpira- 



THE GRAVEL-SLIDE COMMUNITY. 



91 



tion. Perhaps 95 per cent of the rocky soil surface is bare except for 
Parmelia saxatilis and a few other crustose lichens (plate 23). This 
habitat is somewhat similar to that of the sandhills in respect to 
the dry surface mulch and the supply of moisture in the upper layer. 
The root habit is clearly a response to the environment. Undoubtedly 
the large number of roots which have such a strong tendency to run 
up the slope serve in part for anchorage. The distribution of soil 
nutrients may also have some share in this. The bisect in figure 37 
shows that root competition on the gravel-slide is far from severe. 



92 



THE ECOLOGICAL RELATIONS OF ROOTS. 



VII. THE HALF-GRAVEL-SLIDE COMMUNITY. 

The half-gravel-slide community represents a distinct successional 
advance over that of the gravel-sUde, both in the diversity of species 
and the density of plant population, as well as in the correspondingly 
more favorable habitat (plate 25, a). It is an intermediate stage 
between the gravel-slide and the forest. Here the root systems of the 
most important grasses, herbs, and shrubs were studied. 

Elymus triticoides. — ^This grass is perhaps the most abundant and important 
species of the half-gravel-slide community. Because of its excellent root 
development and consequent ability to compete successfully with other 
species, it not infrequently controls large areas, sometimes almost to the 
exclusion of other plants. 

Elymus was excavated at the half -gravel-slide station in coarse, rocky soil 
(plate 25, a, b). The soil of the upper 18 to 22 inches was light brown to gray 
in color. The roots were densely matted. The larger were 1.5 to 2 mm. in 
diameter, being much branched into fine rootlets supplemented by countless 
finer well-branched and matted ones. In fact, the soil to a depth of 15 or 18 
inches was completely filled with a dense network of roots. The larger ones 
penetrated to a maximum depth of 46 inches. The mass of roots at the surface 
had a lateral spread of about 18 inches on either side of the bunch. Many of 
the larger roots descended at an angle of about 30 degrees from the vertical 
to a depth of 15 or 20 inches, when they turned directly downward to a depth 
of 40 to 45 inches. All of these deeply penetrating roots were, like the others, 
extremely well branched to the third and fourth order and ramified through- 
out the crevices of the gravel. Some of these larger roots, after reaching a 
depth of 13 inches, took a course up the steep slope, following at this depth a 
line approximately parallel with the surface of the soil. They were well 
branched, the branches coming up to a distance of only 5 or 6 inches from the 
surface of the ground. The lower part of the root system was white in color 
and extremely fragile. 

Solidago oreophila. — ^The autumn landscape is frequently given tone by the 
masses of yellow flowers of this important half-gravel-slide species. This 
plant consists of clusters of stems connected by short rhizomes, thus forming 
a clump. From the base of the rhizomes arise great numbers of fibrous roots 
about 1 mm. in diameter; as many as 50 to 75 may originate from a single 
inch of the rhizome. Numerous roots run off parallel with the soil surface, or 
nearly so, to a distance of 20 to 30 inches from the base of the plant, sending 
off rather numerous branched and rebranched threadlike laterals from a few 
centimeters to a few inches long and finally terminating in a much-branched, 
brush-like ending. Many of the shallower roots also rim off obliquely, so that 
at a distance of a foot from the plant they may reach to a depth of 8 to 10 
inches or even more. However, these are not so numerous as the superficial 
ones. These shallower roots frequently turn down near their tips to a depth 
of 6 to 15 inches. 

The vertically descending roots send off laterals rather sparingly to a dis- 
tance of 12 or 18 inches, beyond which depth they become more and more 
profusely branched, finally terminating in great clusters of hairlike, minutely 
branched ends. In the more decomposed soils the deepest roots may pene- 
trate to a distance of 30 to 38 inches, while ia the more rocky substratum 
they are much shallower. This deeper group of roots usually spreads laterally 



THE HALF-GRAVEL-SLIDE COMMUNITY. 



93 



to only 6 or 8 inches on either side of the base of the plant. The roots are 
hght tan in color and show considerable tensile strength (fig. 38). 

Rubus deliciosus. — This plant is one of the most important of the shrubs 
of the half-gravel-slide. With Opulaster and certain others it forms the 
transition stage to forest. 

Three specimens were examined which were so similar that only one will be 
described. This plant arose from a tap-root 15 mm. in diameter. It had 3 
main branches, two of which reached a height of 20 inches. At a depth of 3 
inches the tap-root gave off 2 laterals, 2 and 3 mm. in diameter respectively. 
The smaller of these ran off almost horizontally to a distance of 20 inches, 
giving off great numbers of much-branched laterals, the termini being almost 




Fig. 38. — SoUdago oreophila, showing wide-spreading lateral and deep central roots. 

hairhke and reaching a depth of over 20 inches (plate 25, c). The larger 
lateral ran off in the opposite direction to a distance of more than 2 feet, 
branching repeatedly and ending in clusters of rootlets only 4 inches below 
the soil surface. On the first 5 inches of the tap several other smaller rootlets 
occurred, while at 6 inches depth the root divided into 2 equal parts about 6 
mni. in diameter. One of these soon rebranched, while all ran off more or less 
horizontally or obliquely, some to a distance of approximately 3 feet from the 
base of the plant. The maximum depth did not exceed 36 inches. These 
roots branched repeatedly into both large and small laterals, those running 
horizontally being characterized by a multitude of shorter, minutely branched, 
often vertically descending rootlets. Thus the plant is provided with an 



94 



THE ECOLOGICAL RELATIONS OF ROOTS. 



effective absorbing system, which ramifies widely and fills the soil from a 
depth of from 4 to 36 inches. The whole root system is characteristic of the 
half-gravel-sHde root habit. 

Besseya plantaginea. — This plant frequently grows in clusters of 3 or more, 
the individuals of which are connected by short rhizomes about 5 mm. or less 
in diameter and 2 or 3 inches long. The base of the plant and these rhizomes 
are densely covered with fleshy roots about 2 mm. in diameter. As many as 
30 to 40 of these roots occur on a single inch of the rhizome. Many of them 
pursue a vertically downward course and end at a maximum depth of from 
25 to 30 inches (fig. 39). Others run out rather parallel with the surface 
of the soil and at a depth of 2 or more inches to a distance of over a foot, when 
they turn abruptly downward, reaching a depth of 16 or 18 inches. Still others 




Fig. 39. — Besseya plantaginea, showing the widely spreading root system. 

fill in the angle between the horizontal and vertical roots, running off obfiquely 
and then turning downward. Very few branches occur on the first 4 to 6 
inches of the root. But beyond this point the roots (which are now a milH- 
meter or less in diameter) branch freely, sending off laterals to a distance of 
5 or 6 inches, the last 12 to 18 inches being so well branched and rebranched 
as to form a fine absorbing network. Thus the soil within a radius of at least 
a foot from the plant and at a depth of from 20 to 30 inches (except the sur- 
face 2 inches) is rather completely filled with absorbing rootlets of this fleshy 
perennial. As its root system shows, Besseya is an excellent illustration of a 



THE HALF-GRA\^L-SLIDE COMMUNITY. 



95 



transitional form between the gravel-slide type and that of ordinary soil. 
The roots are dark tan in color and are fairly tough and resistant. Five 
plants were examined. 

Geraniuni csespitosum. — This plant is often rather abundant on the half- 
gravel-sHde, where its size and duration give it considerable importance. 
Several plants were examined, which in general were similar (fig. 40). The 
largest had a tap-root an inch in diameter, which at a depth of 3 inches gave 
off a strong lateral more than 1 cm. in diameter. This ran off horizontally, 
following a curved course to a distance of 42 inches, and ended only 18 inches 
below the surface. At a depth of 6 inches the root broke up into 3 parts, all 
of which grew more or less horizontally, none of the branches reaching depths 
of more than 37 inches. Most of these laterals approached the surface, the 
profuse branches often ending within the second to the fourth inch of gravelly 
soil and forming a fine system for surface absorption. Another lateral ran off 
at a depth of about 15 inches to a distance of 52 inches from the base of the 
plant. Geranium also has the same root habit of forming large mats of fine 
rootlets, not only at the ends of the larger branches but also at the extremities 
of the numerous smaller ones. The roots are reddish-brown in color, rather 
brittle, and are uncovered with considerable difficulty. The older roots and 
the crown are frequently more or less decayed. 

Calamagrostis purpurascens. — This plant holds an important place in the 
composition of the half-gravel-slide community. Like Elymus triticoides, 
its root system is so well developed that it can compete successfully with most 
other species (plate 26, b). Two clumps, each about 8 inches in diameter, 
were examined. These were old plants with the flower-stalks 15 inches high 
and in full bloom. From the base of the clmnps almost countless numbers of 
rather tough fibrous roots arose, the largest scarcely more than a millimeter 
in diameter, while many were much smaller. Among them were many new 
roots only a few centimeters long. These ran out in all directions hke the 
radii in a half sphere. The soil was well filled with those that extended verti- 
cally and sHghtly obHquely to a depth of 18 or 20 inches, few of the longer 
roots reaching depths of 32 inches, while the lateral extent of those running 
vertically just beneath the surface was at least 2 feet on either side of the 
plant. Many of the deepest and widest-spreading laterals maintain about 
haff their original diameter for a distance of 12 to 14 inches. From the very 
base of the plant to the extreme tips the supply of fine rootlets is remarkable. 
They branch and rebranch again and again into conspicuous mats of almost 
microscopic rootlets which penetrate every crevice of the gravelly soil. Few 
if any of the grasses examined had such great masses of delicate clusters of 
absorbing rootlets. Such a grass is excellently adapted to live in the half- 
gravel-slide, as more thorough occupancy of the soil can scarcely be imagined. 
In color the roots are bro-v\Ti to light tan. They were removed from the 
gravelly soil with no great difficulty. 

Kceleria cristata. — This grass, which is a characteristic dominant of the 
half-gravel-slide, is distinguished by an extremely fibrous root system. The 
main roots at their outset vary from 0.2 to 0.3 mm. in diameter. At a depth 
of 1 to 4 inches many pf these run off parallel with the surface to a maximum 
distance of 12 inches. These are branched and rebranched to the third 
and fourth order, the ultimate termini being almost microscopic in size 
(plate 26, a). 

Gilia aggregata. — The long pink or red racemes of this abundant biennial 
are very conspicuous on the half-gravel-sUde, where the plant is quite abun- 



THE HALF-GRAVEL-SLIDE COMMUNITY. 



97 



dant. Of the 6 plants examined all had a large tap-root from 4 to 8 mm. in 
diameter. The tap sends off numerous branches near the surface and tapers 
so rapidly that at a depth of 10 inches it seldom exceeds 1 or 2 mm. in diameter. 
The laterals are mostly wide-spreading, frequently nearly horizontal in posi- 
tion, and much forked and rebranched into very abundant fine termini. They 
reach distances of over a foot from the base of the plant. Like most plants of 
this habitat the root system is not deep. No Gilia roots were found below the 
28-inch level, but the moist surface soil is well filled with great quantities of 
fine absorbing rootlets, especially to the depth of 18 inches. 

Potentilla arguta glandulosa. — This species frequently forms families on the 
half-gravel-shde. A large clump of these plants was examined. The com- 
plexity of the rootstocks is such that they are hard to describe. The individual 
clumps are connected throughout long distances by much-branched under- 
ground parts, which often run horizontally or sometimes obliquely at depths 
varying from 3 to more than 18 inches. These underground connecting parts 
may reach a diameter of a centimeter, but are usually much smaller. They 
are very much branched and give rise to the clusters of stems at intervals of 
3 inches to more than a foot. At a depth of 2 to 5 inches the roots often throw 
off a large number of small, short, but exceedingly well-branched surface 
absorbing laterals. The larger roots are usually only 2 to 6 mm. in diameter 
and run off in all directions, some to a maximum depth of 4.5 feet, while the 
surface of the soil is filled with the extremely well-branched brush-like termini 
of the laterals. These clusters are from 6 to 10 inches in length. The deeper 
soils are also completely occupied by the irregularly branched and rebranched 
root network, often to a depth of 4 feet (plate 26, c) . 

Frasera speciosa. — This striking plant starts with a strong, rather fleshy, 
glistening white tap-root, which is 1.5 to over 2 cm. in diameter. It penetrates 




c 


} 














G 











Fig. 41. — Root system of Frasera speciosa. 

vertically downward to a depth of 9 inches, where it divides into 4 branches 
of almost equal diameter. This main tap, throughout its length and even to 



98 



THE ECOLOGICAL RELATIONS OF ROOTS. 



a short distance on the branches, was strongly marked by wrinkles, indicating a 
considerable degree of contraction. Indeed, throughout the entire root system, 
not excluding the smaller branches, characteristic wrinkles and constrictions 
occurred. At the point of origin the branches were about 7 or 8 mm. in diam- 
eter. They traversed the soil in diverse directions, for the most part running 
more or less parallel with the surface, but giving off some vertically or 
obliquely descending branches. The former sometimes ran off laterally to a 
distance of 2 or 3 feet and at a depth of from 6 to 18 inches below the surface 
of the soil. As the ultimate laterals approached their extremities they were 
characterized by a marked, abrupt branching habit, which resulted in a tuft 
of from 6 to 10 or more rather coarse, poorly branched termini varying from 
2 to 5 cm. in length (fig. 41). The diameter of the laterals at the point where 
they develop these tufts is often 2 mm. or more. Arising from these more or 
less horizontal laterals, vertically descending roots were found to penetrate a 
maximum depth of 24 to 30 inches. Like the tap, the branches of all orders 
were more or less fleshy and very brittle, affording a decided capacity for food 
storage. Several plants of different ages were examined. In general the above 
description answers for all. 

Aster porteri. — This plant, which grows abundantly on the half-gravel- 
shde, has the stem clusters connected by short rhizomes and possesses the root 
system characteristic for plants of this habitat. From the base of the stems 
arise a multitude of fibrous roots, the largest seldom being over 2 mm. in 
diameter. Great numbers descend vertically or somewhat obliquely, some to 
a maximum depth of 30 inches. Many others run off rather horizontally or 
obliquely to a distance of 6 to 12 inches on either side of the plant before 
turning downward. All are profusely branched and rebranched into a fine 
absorbmg network, so that every cubic inch of the soil for nearly a foot on 
each side of the plant and to a depth of over 2 feet is forced to yield up its 
available water and solutes to the excellent absorbing system. 

HALF-GRAVEL-SLIDE ROOT SYSTEMS AND THE HALF-GRAVEL-SLIDE 

ENVIRONMENT. 

While the plants of the half-gravel-slide habitat still retain the 
superficial and wide-spreading root habit of their predecessors of the 
gravel-slide, this is supplemented by a much deeper portion which 
extracts water and nutrients below the 18-inch level, as well as very 
largely from the second and third foot of soil. The surface-spreading 
roots may be explained by the frequent mountain showers which 
supply water rather continuously to the shallow soils, while the 
deeper-seated ones reflect the better deep-soil conditions, but especially 
the more intense competition for water of this denser population. 
The thick surface layer of loose rock fragments of the gravel-slide has 
here become more disintegrated and closely packed, and soon gives 
way to soil. Hence water evaporates more freely. This, with the 
competition of the taller half-gravel-slide plants, accounts for the 
disappearance of gravel-slide species. The former shade those of 
lesser height, but especially compete with them for water, undoubtedly 
relying upon their deeper penetrating roots during periods of drought. 



PLATE B 



S C E H B 




le front of the 
fmus triticoides, 
j; H, Heuchera 




Quadral>bisect io the hfllf-gravel slide. The face of the trench was cut along the front of the 
quadrat shown in Plate 26 A: S, Solidago oreophila; A, AUium cemuum; E, Elymus trUicotdes, 
fragments of which are represented in blue; C, Calamagroatis purpurascens; H, Hetuhera 
parvifolia; B, Besseya plantagivea. 



THE HALF-GRAVEL-SLIDE COMMUNITY. 



99 



The soils of the half-gravel-sUde are much more favorable for plant 
growth. Although from one-third to one-half of the surface may still 
be unoccupied, enough plants are present to prevent almost wholly 
the slipping of the soil, even the thin surface layer of pebbles being 
moved between plant clumps only during heavy showers. The first 
8 to 10 inches of soil has a rich brown color, due to the presence of con- 
siderable humus formed by the decayed vegetation. It has many more 
fine particles and fewer large, coarse ones than the corresponding layer 
on the gravel-slide. Although there is considerable variation, the rock 
is decayed to a greater depth, due undoubtedly in part to the excre- 
tions of plant roots and the resultant porosity of the soil, and to greater 
water penetration following the death and decay of the roots. The 
soil underlying the surface layer already described, while still con- 
sisting largely of the decayed granite, is looser in texture and has 
fewer large particles and much more sand intermixed with it, thus 
affording a more congenial home for roots. In the gravel-slide only 
local areas of soil about the sparsely spaced plants are filled with roots, 
especially in the surface layer. In the half-gravel-slide, on account 
of the greater number of plants and especially grasses, all of the soil 
is well filled with roots to a depth of at least 18 to 20 inches or more, 
while many roots penetrate to a depth of 3 feet. 

The amount and distribution of the precipitation is practically 
identical with that of the gravel-slide and the actual available water- 
content is about the same also (table 18). This results from the greater 
water-holding capacity of the soil, due to the presence of more humus 
coupled with more perfect rock decomposition, and occurs in spite of 
the increased absorbing and transpiring surface offered by the plant 
population. The evaporating power of the air as shown in table 19 is 
considerably less than that of the gravel-slide, owing to the greater 
transpiration and shade and the reduced wind movement resulting 
from the denser community. Notwithstanding the fact that one- 
third to one-half of the soil surface may still be unoccupied, root 
competition is rather severe. This is well illustrated in the bisect 
shown in plate B, though this was made in a rather open portion of 
the community. It should be borne in mind that the roots seen repre- 
sent only those actually occurring in a rectangle of soil 7 feet long and 
4 inches wide. Undoubtedly competition is one of the large factors 
in determining root distribution. This must be taken into account 
with the soil conditions in reaching a logical explanation of the root 
habits of the plants of this habitat. 



100 



THE ECOLOGICAL RELATIONS OF ROOTS. 



VIII. THE FOREST COMMUNITY. 



A forest community finally occupies the half-gravel-slide. It is 
represented by Pinus ponderosa or Pseudotsuga mucronataj both of 
which are frequently preceded by a chaparral stage. Along the 
streams and moister slopes, Douglas fir meets the Engelmann spruce, 
Picea engelmanni, with which it often forms a mictium. A rather large 
number of herbs and undershrubs characteristic of the more mesophytic 
type of forest were examained (plate 27, a, b). 

Pirola chlorantha. — This evergreen herb is very abundant and forms 
extensive clans on the floor of the spruce forest. The clusters of leaves arise at 
intervals from a few inches to more than a foot from the glistening white 
underground stems. These vary from 1 to 3 mm. in diameter, branch freely, 
and form a connecting system for the individual plants. They He at a depth 
varying from 0.5 inch to about 5 inches. Just before the rootstock approaches 
the surface to send up a cluster of leaves, it invariably branches, the branch 
continuing to the next plant, etc. The root system is very meager and con- 
sists of brownish roots arising at irregular intervals, usually about 1 inch apart 
on the horizontal rootstock, although it is not unusual to find several iaches of 
the rootstock practically free from rootlets. These roots penetrate the moist 
duff and rich humus soil to a depth of only 6 to 10 inches; while many of them 
are only 1 to 1.5 inches long, others form brush-like clusters 3 to 5 inches in 



Thalictrum fendleri. — This polydemic ranges in habitat from the half- 
gravel-sHde, into which it has worked its way from the bordering woodland, 
to dense spruce forest. In the Douglas fir forest it forms extensive societies. 
The plants here described were examiQed in a spruce forest. The slender 
tops of these plants arise from a meager system of rhizomes ahnost black in 
color, from 0.5 to 2 mm. in diameter, and seldom over 6 to 8 inches long. 
The root system springs from the rhizome near the base of the erect stems, 
sometimes as many as 30 or more fibrous roots originatiag from the base of a 



length (fig. 42). 




Fig. 42. — Pirola chlorantha. 



THE FOREST COMMUNITY. 



101 



single plant. For the most part the roots extend in a direction almost par- 
allel with the surface of the soil. The depth at which they lie varies from 
2 to 8 inches and the maximum lateral extent is 14 inches. Relatively a 
very few of the roots take a more vertical or slightly oblique downward 
course, but none reach a depth below 18 inches. The main roots are a milli- 
meter or less in diameter. Throughout the course of the roots an elaborate 
system of branching occurs. The branches ramify to the third and fourth 
order, and vary in length from a few millimeters to 3 or 4 inches. Near their 
extremities the main roots are so well branched and divided that the tip con- 
sists of a brush-like mat of fine rootlets (fig. 43). 















Jy ... 









Fig. 43. — Thalictrum fendleri, 

Erigeron asper. — ^This plant forms fine societies in the dense shade of the 
spruce-fir forest. The individual clumps are connected by means of branched 
surface rhizomes, seldom over and often less than 2 mm. in diameter, which 
usually do not lie at a depth beyond 2 inches. From the base of the plant in 
particular, as well as along the rhizome, a well-developed system of rather 
fibrous roots arises. The largest of these do not exceed a millimeter in diameter 
and are usually much smaller. While some of these penetrate to maximum 
depths of 26 to 30 inches, the first 18 inches of soil and especially the first 6 
inches are thoroughly filled with the hairlike and profusely branched rootlets. 
Frequently these branches, which arise in great abundance all along the main 
roots, run off obhquely and even almost horizontally to distances of 5 to 10 
inches from the base of the plant (fig. 44). 

Erigeron macranthus. — This plant, which is not infrequent in openings in 
the forest, propagates by means of rather coarse rhizomes often about 5 mm. 
in diameter and several inches in length. From the base of the plant and from 



102 



THE ECOLOGICAL RELATIONS OF ROOTS. 



the rhizomes arise great clusters of fibrous roots from 3 to 4 mm. to only 0.5 
mm. in diameter. While many of these run rather vertically downward, so 
that some of the longer ones reach depths of 40 inches, others spread laterally 
to a distance of 14 to 18 inches and reach depths of only 4 to 12 inches. Thus 
the soil for a distance of more than a foot on either side of the plant and to a 
depth of 2 or 3 feet is well supplied with these fibrous roots. The branching 
of the root is almost identical with that of Erigeron asper (fig. 44), with which 
this species seems to intergrade. 





Fig. 45. — Fragaria virginiana. 



Fig. 44. — ^Rhizome and roots of Erigeron Fig. 46. — Allium cernuum. 

asper. 

Fragaria virginiana. — Fragaria forms societies rather extensively on the 
forest floor, even in the dense shade of spruce seedhngs. In addition to the 
long, slender stolons, it is furnished with rhizomes only 3 to 5 mm. m diameter 
but often several inches long. From these arise the dark-colored fibrous roots 
a millimeter or less in diameter. There are usually 6 to 10 of these on an inch 
of the rhizome. Many of them pursue an obhque course to a lateral distance 
of 4 to 10 inches from the base of the plant, where they reach a depth of 4 to 
12 inches. Others penetrate more vertically downward to a maximum depth 
of 12 to 14 inches. While the first inch of the roots below the superficial 
rhizomes is rather destitute of branches, below this depth, especially in the 
last 6 to 8 inches, the roots are supplied with an abundance of fine rebranched 
termini ranging in length from 1 to 5 cm. (fig. 45). 



THE FOREST COMMUNITY. 



103 



Allium cernuum. — This plant is very characteristic of half-gravel-slides, but 
also occurs rather abundantly in the open portions of the fir forest. The plant 
was examined in the forest habitat. The bulb, which is usually 12 to 15 mm. 
in diameter, occurs at a depth of about 2 inches. From its base arises a cluster 
of 10 to 20, or sometimes more, fibrous roots a millimeter or less in diameter. 
These spread widely in the surface 6 or 8 inches of soil. Laterals were traced 
to a horizontal distance of 8 to 12 inches from the base of the bulb, where they 
ended at a depth of 4 to 8 inches. No roots were found at a greater depth 
than 10 inches. These glistening white fibrous roots branch freely into 
laterals from a few centimeters to 4 or 5 inches in length, but these secondary 
roots are themselves scarcely at all branched (fig. 46) . 

Aralia nudicaulis. — ^This plant is often abundant on the lower slopes of the 
fir and spruce forests, where it sometimes forms extensive communities. The 
individuals are connected by a much-branched system of rhizomes, which 
vary in diameter from 3 to 4 mm. to more than a centimeter. Frequently 
they He just below the soil surface, but they are quite abundant at all levels 
and run in all directions and at all angles in the first 18 to 24 inches of soil. 




Fig. 47. — Rhizomes and root system of Aralia nudicaulis. 

As shown in figure 47, there arise from the nodes of the rhizomes rootlets with 
clusters of well-branched termini, the whole usually not over 2 to 5 cm. long. 
Often, however, rather well-branched roots 18 to 20 inches in length occur. 
The direction in which these run depends somewhat upon the depth of the 
rhizome. If the root arises from a rhizome near the surface it takes a vertical 
or oblique course downward ; at intermediate depths the roots frequently run 
off horizontally, while from the deepest rhizomes it is not unusual to find them 
ascending in an oblique direction toward the surface. As a whole, the root 
system is rather poorly developed, probably due to the dense shade in which 
the plant grows. 

Opulaster opulifollus. — This shrub forms a very characteristic under- 
growth in pine woods as well as in the open fir forest. Indeed, it reaches its 
best development in the better hght of the chaparral community and dies out 
almost completely in forests with a closed canopy. Its role in succession is 



104 



THE ECOLOGICAL RELATIONS OF ROOTS. 



not unlike that of Corylus (page 30). Two plants, 5 and 12 years old respectively 
and about 3 feet high, were examined in the Douglas fir forest. From the base 
of the clump prostrate stems about a centimeter or less in diameter run 
horizontally just beneath the surface, or at least at a depth of only a few 
inches, to distances of 2 or 3 feet, where they give rise to new plants. The 
roots of this plant are often large and woody, sometimes a centimeter or more 
in diameter, but usually smaller. They have a lateral spread of 5 to 8 feet, 
but seldom penetrate the soil to a greater depth than 3 feet (plate 30, a). 
From the base of the clump 6 to 12 of these roots arise. They run out laterally 
in all directions, tapering gradually and branching freely and often dichoto- 
mously, the ultimate branches being very minute and forming a great network 
of absorbing brushes. From the main roots and their branches, as well as from 
the rhizomes, great clusters of exceedingly well-branched laterals (from a few 
centimeters to several inches in length) lay hold upon the moist soil of the 
forest floor. In fact, the majority of the roots are confined to the surface 8 
inches, while only one or two branches were found to reach the maximum depth 
of 32 inches. 

Ribes lacustre. — ^This plant is characteristic of rather dense moist woodland. 
Four specimens, 6 or 7 years old and 1.5 to 2 feet high, were examined. They 
spread through distances of 1 to 2 feet or more by means of rhizomes or 
prostrate stems. From the base of the clump a tap-root 10 to 15 mm. in 
diameter arises. Instead of descending vertically, this almost invariably 
runs off in the surface soil, dividing rapidly into many major branches. While 
many of these are only 2 to 4 mm. in diameter, they taper so slowly that at a 
horizontal distance of 3 to 4 feet from the plant they may still be 1 to 2 mm. 
wide. These long, tough surface laterals pursue their winding way through a 
length of 6 to 8 feet, frequently reaching distances of from 5 to 7 feet from the 
plant. They are usually unbranched or only poorly branched, save for 
isolated brushes of small laterals, until within 12 to 18 inches from their tip. 
Here they divide up into long, slender laterals all well-branched near their 
extremities, but not at all to the same degree as Opulaster. While many of 
these branches end in moist duff, others penetrate downward to a depth of 
8 to 12 inches. The major portion of the root system is confined to the surface 
soil, but a few branches were found at a depth of 2 to over 3 feet (plate 28, b). 

Rosa acicularis. — The rose frequently occurs in the spruce-fir forest, where 
it is a relict of earher stages in the succession. While under the lower light 
intensity of the forest it is merely a low shrub, outside the forest it frequently 
forms dense chaparral several feet in height. The plants are connected by an 
extensive system of rhizomes which lie at a depth of 2 to 10 inches and vary 
in diameter from 2 to 6 mm. These rhizomes run in all directions, branching 
freely and at irregular intervals, giving rise to the stems which seldom reach a 
height in the forest of more than 1.5 to 2.5 feet. The root system originating 
from these rhizomes consists of both long and short roots. The shorter ones 
vary in length from 1 to 12 inches. Almost from their point of origin they are 
profusely branched and rebranched so as to form a brush-like mat. These 
groups of thickly branched roots penetrate the surface soil in all directions, 
forming a very efficient absorbing system. The longer roots vary in diameter 
from 1 to 4 mm. at their point of origin and penetrate the soil downward in a 
wandering course to a maximum depth of 2 to 5 feet. Throughout their course, 
these longer roots are fairly well supplied with both short and long branches, 
the shorter ones varying in length from 0.5 to 3 inches. The longer ones, 
which are almost threadlike in appearance, may wander off in all directions, 
even obliquely upward to a distance often of 2 feet or more. These branches 



THE FOREST COMMUNITY. 



105 



are rebranched much after the fashion of the main roots already described. 
As the main roots and these larger branches approach their extremities, they 
are divided several times so as to form fairly well-branched termini. 

Arctostaphylos uva-ursi. — This ericad, which is the typical undershrub of 
the pine forest, also occm's rather abundantly in the more open portions of the 
Douglas fir forest, where it forms great mats covering areas of many square 
meters. Its xerophytic qualities are shown by its frequent appearance in the 
half-gravel-slide community. A very elaborate and well-developed root 
system arises from the woody prostrate stems, which vary from a few milli- 
meters to 3 cm. in diameter. While some of the roots reach a maximum depth 
of from 40 to 46 inches, the major portion of the absorbing system lies in the 
first 18 inches of soil. From the base of the prostrate stems as well as from 
numerous shallow horizontal roots arise great numbers of rootlets only 1 to 2 
mm. in diameter and from 4 to 8 inches long, but so abundantly supplied with 
masses of branched and rebranched laterals that they fill the surface soil with 
a network of roots. In addition to these, larger branches, sometimes even a 
centimeter in diameter, run off at all angles from the horizontal to vertically 
downward, pursuing tortuous courses and branching and rebranching freely. 
They attain a length of 3 or 4 feet and end at depths varying from a few 
inches to more than 3 feet. The branches from these as well as the ends of 
the main roots themselves form great mats of well-developed rootlets in the 
crevices of the gravelly soil. Plate 28, a, shows something of the extent and 
abundance of these branches. 

Senecio cernuus. — ^This herb is abundant in the fir forest. The root system 
consists of a group of from 8 to 20 white, somewhat fleshy roots. At their 
point of origin they vary from less than 0.5 mm. to 2 or 3 mm. in diameter. 
Relatively few of these penetrate the soil in a vertical or slightly oblique 
direction to a maximum distance of 12 to 15 inches. By far the greater part 
of these roots take a course practically parallel with the surface at a depth of 
from 1 to 3 inches — ^in fact, many of them run just below the surface accumu- 
lation of duff and semi-decayed leaves ; others may take a slightly more down- 
ward course and end at a depth of from 4 to 8 inches below the surface. The 
maximum spread of these horizontal roots may vary from a few inches to 18 
or 25 inches. Throughout their course the main roots are fairly well supplied 
with laterals varjdng in length from a centimeter or less to 6 or 8 inches. 
These in turn may be branched again to the second and third order, so as to 
form a very efficient surface system well adapted to absorb the shallow water- 
content resulting from the frequent mountain showers. Eight plants were 
examined (fig. 48). 

Castilleia miniata. — This plant is a conamon component not only of grassy 
half-gravel-slides, but it also occurs rather abundantly in the fir forest. Rela- 
tive to the size of the plant it has a very meager root system. This consists 
of 5 to 20 main branches, varying in size from 1 to 3 mm. These all show a 
strong tendency to spread out laterally and run off in the surface soil, usually 
at a depth not greater than 5 to 6 inches to distances of 12 to 18 inches or less. 
No roots were found penetrating deeper than 8 inches. While these rather 
coarse, yellow, brittle roots are fairly well branched, they are only poorly 
supplied with fine absorbing laterals as compared with other plants examined 
in this community. 

Heuchera parvifolia. — This plant is fairly abundant in the Douglas fir 
forest. It has a strong tap-root, sometimes as large as 15 mm. in diameter. 
However, this tapers so rapidly that within a distance of 6 to 8 inches it is 



106 



THE ECOLOGICAL RELATIONS OF ROOTS. 



usually not more than 1 to 2 mm. wide (fig. 49). Of the 10 plants examined 
only 3 had roots penetrating deeper than 6 inches. In these cases the tap 
penetrated rather vertically downward, having no large branches but being 
covered below a depth of 4 inches with multitudes of fine laterals only a 
centimeter or two in length. Near the tips they branch profusely and reach 
depths of 15, 18, and 20 inches, respectively. Even in these plants the 2 or 3 





\ 


» A 

i 

/ 








I 


r 

/ 




f 




V 





















Fig. 48. — Senecio cernuus. 

strong laterals, 4 to 7 mm. in diameter, originated just below the crown and, 
tapering rapidly like the tap-root, ran off horizontally at a depth of about 2 
inches to a distance of 18 to 24 inches from the base of the plant. Unlike the 
tap, these large laterals were well branched with rather widely spreading 
rootlets, which gave off multitudes of tertiary branches, all of which ended in 
a great network of absorbing rootlets. Many fine, much-branched rootlets, 
only a few inches long, also originated from the crown of the plant where it 
was buried in the moist duff. The root systems of those described were the 
same as the others examined, except that the tap-roots of the latter group 
took a course up the slope and assumed the r61e of laterals already described. 

Saxifraga bronchialis, — ^This plant forms large mats, often several feet in 
extent, which cover the moist soil on the forest floor or even occur on moist 
rock surfaces. It is especially well developed in the Douglas fir community. 
From the main root prostrate stems from 1 inch to over 1 foot in length extend 



THE FOREST COMMUNITY. 



107 



out in all directions in the surface soil. These send up erect branches 5 inches 
long, the shorter ones being near the growing tip. As the soil accumulates 
about the bases of these erect stems, the lower leaves die and only an inch or 
less of the living tip is exposed. These branches with their dead leaves and 
accumulated debris occur in such density that they afford a congenial sub- 
stratum for the abundant rootlets which arise from the horizontal portion 
especially, as well as from the erect part of the stems. These delicate roots, 
however, which vary from 1 to 14 inches in length, run off in a horizontal 
direction in the moist soil just below the mat and seldom reach a depth greater 
than 3 to 4 inches. They divide into fine, hairlike, well-branched termini and 
occur in exceedingly great numbers. The main root or roots, depending upon 
the size of the mat, are tough and somewhat woody, black in color, and from 
2 to 3 mm. in diameter. They also run off horizontally and usually up the 
slope, branching profusely and often dichotomously. They spread out some- 
what fan-shaped and reach a distance of from 3 feet to over 5 feet from the 
base of the plant. Although they were found to penetrate not deeper than 
4 to 5 inches, their wide spread and the great numbers and extreme division 
of their branches enable them to extract sufficient water from the surface soil. 
Indeed, the termini of many of these branches consists of such a mat of deli- 
cate, almost microscopic, ultimate branches that it is quite impossible to depict 
them adequately either with the pen or the camera. 




Fig. 4Q.—Heuchera parvifoUa. Fig. 50 —Haplopappus parryi. 

Haplopappus parryi —This composite forms dense societies in the shady 
woodland, the rosettes of long leaves almost completely covering the surface 
over local areas (plate 27, b). It is supplied with a system of stout, much- 
branched rhizomes that vary in diameter from 2 to 8 mm. and extend from 
just beneath the surface to a depth of from 6 to 8 inches (fig. 50). From these 
rhizomes arise large clusters of roots ranging in diameter from 2 mm. to less 
than 0.5 mm. These roots descend rather vertically or somewhat obhquely 
and completely fill the soil to a depth of 25 inches, while some reach a maxi- 
mum depth of 28 inches. The younger roots are supplied with many branches 



108 



THE ECOLOGICAL RELATIONS OF ROOTS. 



from their point of origin and are reenforced by a great network of branches 
arising from the older roots in the deeper soil at 6 to 10 inches. The deeper 
roots and their network of fine branches spread widely, often running for 
several inches horizontally through the moist gravel soil. The whole root 
system is surprisingly well developed, considering the habitat conditions 
under which the plant grows. 

FOREST ROOT SYSTEMS AND THE FOREST ENVIRONMENT. 

A survey of the preceding data reveals the fact that the herbs and 
shrubs of the forest floor are relatively shallow-rooted. Almost with- 
out exception the bulk of the absorbing system lies within the surface 
18 inches of soil. Arctostaphylos and Rosa have roots which penetrate 
deeper. The former, however, shows its xerophytic tendencies by 
being the typical half-shrub of the open pine forest and often forming 
large mats in the half-gravel-slide. In the Douglas fir forest it occurs 
only in the most open places and disappears entirely in the spruce. 
Rosa is a relict from an earlier stage of succession. It makes a much 
better growth in full light, even in dry situations, where it forms dense 
thickets. Even the roots of many of the Douglas fir and spruce trees, 
including seedlings, saplings, and mature plants which were inciden- 
tally encountered in the course of the work, were found to possess 
many shallow roots (plate C ) . The root relations of forest dominants 
offer an exceedingly profitable field for investigation. 

The water-content of the soil offers the logical explanation for the 
shallow root development of these forest plants. The forest soil is 
usually deeper and always much richer in organic matter than that 
of the half-gravel-slide. Usually there is a layer of 1 or 2 inches of 
duff, beneath which the soil is very rich in decomposed humus to a 
variable depth of 8 to 18 inches or more. Such a substratum furnishes 

Table 20. — Average daily evaporating power of the air in the forest communities. 



Date. 



June 10-17 

June 17-24 

June 24 to July 1 

July 1-8 

July 8-15 

July 15-22 



Douglas 
fir. 



c.c. 
28.8 
11.1 
20.4 
15.2 
5.7 
6.1 



Engelmann 
spruce. 



c.c. 
21.6 

7.1 
15.0 
11.1 

4.4 

5.2 



Date. 



July 22-29 

July 29 to Aug. 5 

Aug. 5-12 

Aug. 12-17 

Aug. 17-23 



Douglas 
fir. 



c.c. 
11.3 
16.0 
8.6 
8.5 
11.6 



Engelmann 
spruce 



c.c. 
8.5 

11.4 
6.2 
6.3 
8.8 



an excellent medium with a high water-holding capacity to catch the 
precipitation of winter as well as that of the frequent summer showers. 
The shade of the trees and various forest layers reduces the evaporat- 
ing power of the air (table 20), while the water-loss from the soil is 
further retarded by the layer of duff. 



WEAVER 



H R Pi F H 




Quadrat-bisect showing root systems of shral 
the front edge of the quadrat shown in p 
Pi, Pinus flexiUs; F, Fragaria virginiana; 



H R Pi F H F H F H H C 




Qufldrat^bisect showing root syetemB of shrubs and herbs of the forest floor. This was made along 
the front edge of the qua^iat shown in plate 27 B : H, Haplopappus parryi; R, Rosa adadaTis; 
Pi, Pinus fiexiUa; F, Fragaria virginiana; C, Chamasneriumarigustifolium; V, Picea engelmanni. 



X 



THE FOREST COMMUNITY. 



109 



That sufficient water ordinarily does not enter the soil to penetrate 
deeply is shown by the following series of soil-moisture determinations 
made during the summer of 1918: 



Table 21. — Water-content of the soil in the forest communities. 



Date. 


Depth of sample, 
to 6 inches. 


Depth of sample, 
6 to 12 inches. 


Depth of sample, 
12 to 18 inches. 


Depth of sample, 
2 to 3 feet. 


Fir. 


Spruce. 


Fir. 


Spruce. 


Fir. 


Spruce. 


Fir. 


Spruce. 


June 10 


6.6 

5.3 
19.8 
13.1 

8.1 
14.7 

9.2 
16.4 
16.5 
*18.0 
16.4 


12.7 
8.5 
19.2 
15.3 
12.8 
15.1 


3.1 
2.1 
6.2 
7.3 
5.4 
7.1 
3.9 

11.3 
7.0 

*9.1 
7.8 


8.1 
7.2 
10.0 
5.2 
5.2 
8.9 










June 15 

June 24 


1.7 
3.5 
5.8 
4.8 
4.6 
2.7 
8.6 
6.2 
*6.7 
3.2 


5.6 
9.2 
4.4 
8.4 
5.1 


1.7 


4.6 


July 1 






July 8 






July 15 

July 23 


3.6 


7.3 


July 29 


18.8 
16.3 
10.3 
21.0 


9.2 
7.7 
7.0 
11.2 


5.8 
7.7 
4.3 
8.2 












Aug. 12 

Aug. 19 


*5.6 


6.9 









* Samples taken the day following a rain of 1.84 inches. 



An examination of table 21 makes clear the fact that the greatest 
amount of available water is in the surface 18 inches of soil. This 
should account for the shallow root habit of forest herbs and shrubs, 
"^^e lesser extent of the root systems of these forest plants, when com- 
pared as a group with the species of any of the preceding habitats, is 
correlated with the more favorable aerial conditions for water con- 
servation on the forest floor. Indeed, not only is the temperature 
miich lower, but the wind movement also is greatly checked. In the 
fir forest the average daily wind velocity was only 29 miles as com- 
pared with 67 miles on the half-gravel-slide. Likewise, in the forest 
diffuse light and low evaporating power of the air are the rule. Com- 
pared, for example with the half-gravel-elide habitat, the aerial con- 
ditions in the Douglas fir forest are only 54 per cent as severe. The 
evaporation in the spruce forest is only 40 per cent as great as that on 
the half-gravel-slide. 



110 



THE ECOLOGICAL RELATIONS OF ROOTS. 



IX. ECADS. 

During tihe course of these investigations, a number of species were 
encountered and excavated in two or more different habitats. While 
a few of these have already been described in explaining the root 
habits of plants of the several plant communities, others have been 
reserved for comparison in this place. Differences in ecads were 
always determined by actual comparison of plant materials from the 
two habitats at the same time, and these differences are expressed 
wherever possible by means of photographs or drawings. Any personal 
error was further checked out by the judgment of a second person, 
no statements being made until there was a consensus of opinion. 

Smilacina stellata. — The shade form was excavated in a spruce-fir forest 24 
feet from the brook bank and 3.5 feet above the water-level. The ground was 
covered with about 0.5 inch of raw humus underlaid with about 2 inches of 
well-decomposed humus mixed with sand. Below this was very moist sand 
with gravel and rocks. The light value was only 0.02. 

The roots were much sparser and shorter, although somewhat greater in 
diameter than those of the gravel-slide form (page 85). While an inch of the 
rhizome of the latter form gave rise to 18 roots averaging 3.5 inches long, an 
average inch of the shade rhizome showed 8 roots only about 1.5 inches in 
length (plate 29, a) . The longer roots, which have their origin near the base of 
the vertical stems, were Hkewise both fewer and shorter. In the gravel-sUde 
form they varied from 2 to 5, in the shade form there was frequently none and 
seldom more than 2. The maximum length of the latter was only 16 inches, 
with an average length of about 11 inches; in the former case roots were 
traced to 44 inches in depth, with an average depth of about 32 inches. An 
intermediate stage was found in the fir forest, a habitat which has been shown 
to be somewhat less meosphytic than the spruce. The roots were nearly as 
sparse as those in the spruce forest but somewhat longer. 

Chamsenerium angustifolimn— This cosmopolitan plant is found in habitats 
of all degrees of mesophytism, from the gravel-shde and bare bum to very 
moist dark places in the spruce forest. A large group of these plants was 
examined at the foot of a slope near a stream and 4 or 5 feet above water- 
level. They were growing in the haK shade of Douglas fir and mountain 
maple on one side. Below the first inch or two of duff and humus the soil 
consisted of a rich black sand intermixed with gravel and rich in humus to a 
depth of 2 or 3 feet. The plants are connected by means of strong, tough, 
well-branched rhizomes from 5 mm. to more than 10 mm. in diameter. These 
lie at depths varying from only 0.5 inch to 6 or 8 inches. The subterranean 
portions of the erect stems are practially free from roots. At rather distant 
intervals along the rhizomes, roots varying from 1 to 5 mm. in diameter 
arise. Many of these pursue a more or less vertically downward course, with 
but a few major branches, and reach depths of 35 to 48 inches (fig. 51). They 
taper very gradually and are only fairly well clothed with small rootlets. 
The latter usually occur in clusters of from 3 to 7 and are frequently less than 
an inch long. In addition to these, however, other longer and abundantly 
branched laterals supplement the absorbing system. These extend from 3 
to 10 inches. Sometimes these deep roots break up into numerous branches, 
all of which are similar to those aheady described. In addition to these larger 
roots, the rootstock also gives rise to smaller, shallower, and frequently 



ECADS. 



Ill 



obl que laterals. In fact, the larger roots also sometimes take an oblique 
course. As indicated in the figure, the root system is somewhat meager. The 
older roots may be identified by their brownish-yellow and somewhat flaky 
cortex and the tough, rope-hke stele within, while the younger roots can be 
told by the clustered arrangement of their branches. 





r 






-') ' 














^^■^ J A 




0. 










> 


















1 









Fig. 51. — Root systems of ecads of Chamcmerium angustifolium: a, gravel-slide; 6, forest. 

A number of plants were examined on a steep gravel-slide. Like those 
in the fir forest, thej^ were mature and in full bloom. Those on the gravel- 
sUde ranged from 1.5 to 2 feet in height, while the shade forms were 4 to 5 
feet tall. The most noticeable differences in the underground parts were 
the shallowness and linear extent of the root systems and the remarkable 
difference in number and fineness of the smaller roots. Of the several plants 
examined on the gravel-slide, all sent roots up the slope at a depth of 2 to 
10 inches or more. These frequently ran rather parallel with the surface, 
often branching iuto major branches, for a distance of 3 to 6 feet or more. 
From these rather shallow roots some of their branches pursued an obliquely 
downward course. None of them reached a depth greater than 20 inches, 
but almost invariably turned and ran parallel with the surface of the soU. 

A comparison of the two root systems emphasized the paucity of small 
laterals in the forest ecad and their abundance in the gravel-slide. In the 
latter habitat, great clusters of fine rootlets (resulting from the repeated 



112 



THE ECOLOGICAL RELATIONS OF ROOTS. 



branching of the main roots, especially near their extremities) filled the soil 
with a web-like network in marked contrast to the rather sparse, absorbing 
rootlets of the shade form. 

Elymiis triticoides. — family of Elymus triticoides, which formed a sod 
completely covering the ground under a canopy of Douglas fir, was examined. 
The soil was fairly well decomposed and filled with humus to a depth of 18 
inches. The underljdng soil was very similar to the second foot of soil in the 
half-gravel-shde. A trench 4 feet long was dug through the midst of the com- 
munity and careful examination showed that no roots penetrated to a depth 
greater than 38 inches, while in the haK-gravel-slide a maximum penetration 
of 46 inches by the larger roots was found (page 92). A marked difference was 
evident both in the density and coarseness of roots, especially in the first 2 
feet of soil. In the half-gravel-shde the roots were not only much coarser, but 
they also filled the soil much more completely. In the shade form the flowering 
staflis were also much less abundant. The ecads of this species show differ- 
ences in depth of penetration and in the number of roots per given area, as 
well as in size. 

Bouteloua gracilis. — This widely distributed and important grass has been 
studied under four different habitat conditions. In the prairies of eastern 
Nebraska it occurs often in fairly pure stands on the fighter soils of gravelly 
ridges and is also frequently found dominating areas of alluvial soil on bottom 
lands. One of the several plants obtained from a gravelly ridge at Belmont 
is shown in plate 6, b, while the root system is fuUy described on page 8. A 
few roots reached a maximum depth of 46 inches, although below 2 feet they 
were very sparse. Great masses of roots occupied every cublic centimeter of 
the soil to a depth of 18 inches. Other groups of plants examined in two 
locations on alluvial soil showed a somewhat poorer development of the root 
system, but the general distribution and depth were very similar to those 
growing in the gravelly soil. 

This grass was again examined on the plains of Colorado, where Bouteloua 
is the dominant, but where societies of Aristida purpurea, Psoralea tenuiflora, 
and Artemisia frigida, with other deep-rooted plants, indicate rather favorable 
water-content conditions. Here the soil was found to be weU filled with these 
rootlets to a depth of 30 inches, while in the next 6 inches they were still fairly 
abimdant, some of the longer ones penetrating to a maximum depth of 48 
inches. The lateral spread in the surface soil was much more pronounced than 
in the plants examined near Lincoln. The plains species have msmy wide- 
spreading and exceedingly well-branched roots filhng the surface soil and 
extending to 1.5 feet or more on all sides of the clump. 

This grass was likewise studied in the typical hard-land or short-grass 
country about 25 miles southeast of Colorado Springs (p. 65, plate 20, a), 
where Bouteloua again dominates and the shallow-rooted Opuntia polycantha 
is the principal species, a community conspicuous for the almost entire 
absence of Psoralea, Gutierrezia, Chrysopsis, and other deep-rooted dicotyle- 
dons. A number of roots were found to penetrate to a depth of 51 inches, 
v/hile they were very abundant to a depth of 40 inches. Their wide-spreading 
habit and their position in the soil was almost identical with those ui the plains 
habitat just described. 

Finally, a fourth group of these plants was excavated in the sandhills 
(p. 68), about 40 miles southeast of Colorado Springs but under the same 
general condition of rainfall. A trench 8 feet long and 5 feet deep was dug on 
a well-covered sandhill where this species was the dominant and formed 
rather pure stands. From the short, tough rhizomes, usually only an inch or 



ECADS. 



113 



two in length, great numbers of tough, fibrous roots a milHmeter or less in 
diameter arise. On 4 cm. of one of these rhizomes 53 roots originated. While 
some of these pursue a rather vertically downward course to a maximum 
depth of 27 inches, or wander out somewhat obliquely, great numbers, perhaps 
half, run off more or less horizontally with the surface of the soil, reaching 
distances varjdng from 10 to 25 inches on either side of the clump (plate 29, b). 
Some of these ended only 2 to 4 inches below the soil surface, the tips of the 
growing roots being characterized for a distance of 1 to 3 inches by the entire 
absence of lateral branches. In fact, some of these had penetrated so near the 
surface that the soil had dried out and the root-tips had died. However, on 
mature roots, not only are the tips but also the entii-e root system densely 
clothed with great masses of capillary, much-branched rootlets of varying 
lengths, exclusive of the first 2 to 4 cm., which lie in the dry sand. Most of 
these branches are only 1 to 3 cm. in length, but frequently the main fibrous 
roots, especially the deeper-lying ones, repeatedly give off laterals many 
inches long, which run off at various angles from the course of the main root. 
While the first 8 inches of soil are hterally filled with great masses of this 
wonderfully efficient absorbing system, the lower soil stratum to 2 feet is also 
abundantly supplied with fine roots. 

The marked difference in the root penetration of the sandhill ecad as con- 
trasted with those of the plains may be explained upon the basis of a fairly 
abundant water-supply in the shallower soils (first 2 feet) of the sandhills as 
contrasted with the uniformly drier soils of the plains. As pointed out on 
page 79, the surface sand forms a splendid mulch which inhibits water-loss by 
evaporation. It may be noted that the plants from the gravelly soils of the 
knoll in the prairie near Lincoln are intermediate in root depth between the 
sandhiUs and the plains ecads. 

Stipa comata. — As described on page 53 and illustrated in plate 19, a, this 
species is deep-seated in the hard, dry plains soil. The surface 28 inches of 
soil is thoroughly occupied by the main roots, which are only a few millimeters 
apart, the interstices being completely filled with horizontal branches. Even 
to a depth of 32 inches the soil is fairly well filled with much kinked and 
rebranched threadlike rootlets, while not a few reach a maximum depth of 
over 5 feet. In addition, the lateral spread of the shallower oblique roots is 
18 inches or more on either side of the base of the clump. The sandhill ecad 
of this species, while having a similar surface-root distribution, stands out in 
marked contrast as regards its superficial position in the soil, a condition 
already explained upon the basis of a relatively high water-content of the 
shallow soil. 

Tufts of Stipa comata about an inch in diameter were abundant on the slopes 
and tops of captured sandhiUs. Of the 5 clumps examined, rather large roots 
(to the number of 15 to 25) originated from the base of the crown. None of 
these exceeded a milhmeter in diameter. While some of them descended rather 
vertically to somewhat obliquely, a few reaching a maximum depth of 23 
inches, the remainder ran off much more obhquely, having a lateral spread of 
10 to 15 inches and reaching depths varying from 3 to 20 inches. Although 
the roots are not branched in the first 1 or 2 inches of soil, they are densely 
covered with hairs. Upon entering the moist soil they immediately begin to 
branch, gi^dng off countless numbers of very fine, mostly rebranched short 
laterals only a few millimeters to an inch in length. At a distance of from 8 
to 12 inches from the tip, the main roots frequently break up into 3 to 5 
laterals several inches in length. These spread out in all directions and their 
profusely branched and rebranched network of finer absorbing rootlets lays 
hold upon considerable areas of soil. 



114 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Yucca glauca. — The root systems of numerous individuals of this species 
were examined in the plains soil (page 51) and also in the half-gravel-slide of 
the mountains. The main roots, which are more abundant in the haK-gravel, 
slide ecad, are much more profusely branched. Great clusters of long, well- 
branched sublaterals fill the soil in sharp contrast to the poorly branched 
laterals characteristic of the half-dozen yuccas examined on the plains (fig. 
52). A marked difference was also noted in the diameter and the fleshiness 
of the system of roots. While most of those of the plains were rather dry and 
often papery, those of the gravel-slide were generally more turgid, thicker, 
and fleshy. The rhizome system is somewhat more branched and is nearer 
the surface than on the plains. 



Fig. 52. — Fragments of the roots of Yucca glauca: a, plains form; &, half-gravel-slide form. 

Allionia linearis. — This four-o'clock is a widely scattered secondary species 
on the plains and also occurs rather abundantly in the sandhills. Of the 8 or 
more plants examined in the former situation, all had strong and rather 
fleshy tap-roots, the largest 12 mm. in diameter. However, the tap is not 
uniform, often contracting at the top and being largest at a depth of from 8 to 
16 inches. At about 2 feet in depth it narrows down abruptly to scarcely more 
than 2 mm. Except for small curves and kinks, it pursues a vertically down- 
ward course, some to a maximum distance of 5.5 feet. The first 18 to 24 
inches of the root are practically unbranched, and in fact the lower part of the 
root system has no long laterals. Those that do occur at irregular intervals 
consist of groups of hairlike laterals, seldom more than a centimeter in length 
and usually only 4 or 5 mm. long. These are very minute in diameter and 
rather well branched. The tip ends abruptly with only a few short branches. 
The root is dark brown in color and very brittle, the interior parts being 
glistening white. At a depth varying from 6 to 14 inches, usually one or 
sometimes two laterals arise. These may be as large as 2 mm. and run off 
horizontally or shghtly upward, ending 18 inches to more than 2 feet from 
the base of the plant. Like the main root, they are only poorly clothed with 
clusters of short branches (fig. 53). 

Other specimens were examined in the sandhills from a half-captured 
blowout. Five roots were examined. In general they were very similar, the 
strong taps (which were about 5 to 8 mm. in diameter) descended rather 
vertically to a depth of 4 or 5 feet. A typical specimen is shown in figure 54. 
One to three branches, usually only one, occurred in the first foot of soil. The 
lower portions of the roots had a few branches or were in some cases almost 
destitute of them. The tops were both greater in diameter and taller than the 
plains specimens examined. The leaf surface also was greater. 




ECADS. 



115 



From the above, as well as from figures 53 and 54, it may readily be seen 
that Allionia linearis is a rather stable species as to root system, the root habit 
differing only shghtly in the two habitats. The clusters of small branches 
were quite pronounced in the plains ecad, while this was compensated for in 
the sandhill form by a somewhat greater degree of branching. The habit of 
sending off one or more large and rather horizontal branches in the surface 
foot of soil was characteristic in both habitats, as was also the strong tap- 
root, which showed about the same degree of penetration in both groups. 




Fig. 53. — Allionia linearis from the Fig. 54. — Allionia linearis horn the 

plains. sandhills. 

Abronia fragrans. — ^This plant is rather widely distributed over the plains 
in disturbed areas, where, as in the sandhills, it frequently forms socies. The 
large, showy, hemispherical clusters of flowers make it very conspicuous. 



116 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Three plants were examined. The description for one will answer in general 
for all. The two largest had tap-roots 6 and 7 mm. in diameter respectively; 
the smaller was only 3 mm. in diameter. Both of the larger plants had 3 
stems which reached a height of 14 to 16 inches and were either in blossom or 
in fruit; hence these may be considered typical mature plants. The tap-roots, 
except where they twist and curve irregularly back and forth for an inch or 
more in crevices of the cloddy soil, pursue a vertically downward course. The 
smallest reached a depth of 34 inches, the largest 40 to 43 inches respectively. 
As a whole the root is very poorly branched, no large laterals being given off. 
Beginning near the surface, however, and extending quite to the tip, short 
threadhke laterals, seldom exceeding 0.2 to 0.3 mm. in diameter and often 




Fig. 55. — Ahronia fragrans from the sandhills. 

arising in groups of 2 or 3, run out laterally for a distance of less than 1 cm. 
to more than 4 inches. The longer ones occur within the sm-face foot of soil. 
These branches are very scattering, often 3 or 4 cm. of root ha\dng no branches 
whatsoever. The tip is only poorly branched. The older roots are black in 
color, this being confined to the outer layer of cortex, the interior being white. 
The younger roots vary in color from brown to yellowish. They are removed 
only with extreme difficulty from the hard soil. The tiny lateral rootlets are 
seldom branched beyond the second order and do not form a network (plate 
29, c). 

Foui' more rather large plants, all in flower or fruit, were examined. They 
were growing on a captured sand-dune and had tap-roots varying from 6 to 
10 nun. in diameter. One of the largest of these is shown in figure 55. The 
tap is prominent throughout and tapers slowlj^ to the tip, which reaches a 
depth of from 40 to 48 inches. The plant is characterized by 2 tjjyes of 
branches; the shorter ones vary from 2 to 20 mm. in length and occur in tufts 



ECADS. 



117 



or clusters, both on the main root and larger branches. Besides these, all 
of the roots examined had 3 to 6 or more larger branches, which ran off from 
the tap somewhat horizontally at all levels and to distances varying from a 
foot to over 3 feet. As shown in figure 55, the root system as a whole 
is poorly branched, although the sandhill ecads were much better provided 
with laterals and smaller rootlets than were the specimens examined in the 
plains soil. The root is yellowish-brown to nearly black in color. It is very 
brittle, the laterals especially being followed with extreme difficulty. 

A comparison of these descriptions, together with an examination of the 
two figures, reveals the fact that the sandhill form, while showing the same 
habit of a strong tap-root which penetrates to about the same depth as the 
plains species, differs in its more abundant branching as well as in the great 
lateral spread of the branches. In these respects the modifications are not 
unhke those of other sandhill ecads already described. 

Kceleria cristata. — This very important and cosmopolitan grass was first 
examined in the prairies of eastern Washington, where it is a common bunch- 
grass. Under the prevailing conditions of rather moderate precipitation 
(about 21 inches), most of which falls in winter and is followed by a season of 
simmier drought, it flowers in late June, soon dries up, and remains dormant 
imtil revived by the autumn rains. In this region the deepest root found w£is 
at 28 inches and 15 inches was determined as the average maximum depth. 
In the prairies of eastern Nebraska it is also an important grass, growing under 
an annual precipitation of 28 inches, much of which falls during the growing 
season. Here none of the roots of the 7 plants examined reached depths of 
over 21 inches, while the average maximum root depth was about 15 inches. 

Kceleria was further examined in the hard plains soils of east-central 
Colorado and under an annual (summer) precipitation of about 15 inches. 
Here the root distribution was almost identical in lateral spread and depth 
with those already described. The greatest depth was 26 inches and the 
average maximum depth about 14 inches. Finally, the same species was 
excavated on a half-gravel-slide at an elevation of 9,000 feet in the Rocky 
Mountains. As was characteristic in the other habitats, the first 6 to 8 inches 
of soil was completely filled with the profusely branched and matted root 
system. Roots were quite abundant to a depth of 12 inches, while some of 
the longest penetrated to 18 or 20 inches. 

A comparison of plates 5, a, and 26, a, showing the roots of plants from the 
prairies of eastern Nebraska and from the gravel-shde respectively, reveals 
the fact that they are almost identical. Compared with the profound differ- 
ences in the ecads of Stipa comata and Elymus triticoides, Kceleria is a remark- 
ably stable species. 

Chrysopsis villosa. — This composite is widely distributed throughout much 
of the grassland formation, occurring abundantly in the sandhills (plate 21, a). 
On the plains several plants were examined. All had strong tap-roots, which 
were woody for 2 or 3 feet. The height of the plant above ground was 6 to 10 
inches. A second plant had a tap-root with a diameter of 12 mm. It pene- 
trated almost vertically downward, except for local curves and kinks, usually 
less than an inch from a vertical line, to a depth of 8.6 feet (fig. 56). The first 
18 inches of the tap, beginning about an inch below the soil surface, is abun- 
dantly supplied with threadlike laterals, mostly less than a milhmeter in 
diameter, which run out in a horizontal direction from a distance of 1 inch to 
more than a foot. These laterals are practically free from branches. At a 
depth of 6 to 18 inches, half a dozen stronger laterals, each about 1 mm. in 



118 



THE ECOLOGICAL RELATIONS OF ROOTS. 



diameter, ran off rather horizontally for a distance of 18 inches or so before 
turning downward. At 2 feet the root had a diameter of only 2 mm. and was 
very much twisted and curved, as is characteristic of many plants in pene- 
trating the hard soil. Practically no branches were given off to the very tip, 
which was threadhke and unbranched. At a depth of 6 feet, where the soil 
became looser and more moist, the roots pursued an even course, in striking 
contrast to the twisted parts in the harder soil. The younger portion of the 
root is almost white in color and covered with a dense coat of root-hairs. The 
older parts are dark brown. This description answers for two other plants 
which were growing rtear it. 

Another plant ga . off two laterals at a depth of 18 and 21 inches respec- 
tively. These were ea^h a millimeter in diameter and ran rather horizontally 
to a distance of 2 and 2.5 feet. At a depth of 34 inches another lateral, 2 mm. 
in diameter, came off and ran almost parallel with the tap. This plant had a 
woody tap 3 cm. in diameter, which spht at the top and gave rise to three 
clusters of stems. It tapered rapidly, however, and at a depth of 2 feet was only 
4 mm. in diameter. The main root outside of branches already mentioned 
branched but Uttle, as indicated in the drawing. It reached a depth of 13 feet. 

On a half-captured dune in the sandhiUs, 5 or 6 large mature plants were 
examined, aU of which agreed in having a multicipital crown arising from 
the 2 to 5 upper forks of the thick, woody tap-root. The surface branching 
was exceedingly well developed. From the very base of the plant numerous 
lateral branches arose, varying from 0.5 mm. to 6 or 8 mm. in diameter. All 
were fairly well rebranched, the smaller laterals and the larger branches of 
the major ones being densely clothed with root-hairs. In most cases the 
tap-root, owing to the enormous numbers of large lateral branches, rapidly 
diminished in size, so that at a depth of 18 inches none were larger than 5 
nam. in diameter. This stands out in marked contrast to the stout, deep- 
seated tap of the plains. Indeed, the deepest root examined penetrated to a 
depth of only 6 feet 11 inches, while most of them reached depths of only 
4 to 6 feet. The lateral branches, both large and small, formed a dense 
network of roots filling the soil from 18 to 24 inches on either side of the 
plant. AU of the larger roots were weU suppUed with small, well-branched 
absorbing laterals. The divisions of the tap-root were themselves branched 
and rebranched to such an extent that, as they passed downward, great 
numbers of roots were found penetrating the soil to depths of 4 or 5 feet. 
Still other plants were examined which confirm the above descriptions, 
the taps and larger branches having a strong tendency to run obhquely or 
even horizontally, rather than vertically downward as in the plains ecads 
(plate 30, b). 

This affords a very clear case of the effect of the habitat upon root develop- 
ment. The plains form has a root which is approximately twice as deep- 
seated as those in the sandhills. While the tap is prominent throughout in 
the former, it soon loses its dominance in the sandhiU form and often scarcely 
exceeds in importance some of the stronger laterals. While both forms are 
supphed with rather abundant surface laterals, in the plains form these are 
largely confined to the surface foot, while in the sandhills they occur to a 
much greater depth and are abundant along all of the major branches. The 
branches in the first foot of the plains soils were mostly less than 1 mm. in 
diameter and had a horizontal spread seldom exceeding 18 inches. They 
were practically free from branches. In the sandhiUs the laterals ranged 
from 0.5 to 8 nun. in diameter and were rather well branched. In number 
these lateral branches, both large and small, formed a dense network of roots 



ECADS. 



119 



filling the soil to a distance of 18 to 24 inches on either side of the plant. 
Such marked differences in root habit are probably accompanied by marked 
changes in the aerial parts. This is a rich field for future investigation. 

Euphorbia montana. — This spurge is rather widely distributed throughout 
the plains and mountains and is often locally abundant. Those examined 
on the plains had a strongly marked tap-root, which started with a diameter of 
6 mm. and pursued a tortuous and zigzag course downward to a depth of 
7.5 feet, where it ended in pearly white, hairlike branches (fig. 57). Within the 
first foot there were 4 short branches from 4 to 8 inches long, which were 
themselves shghtly branched. The larger branches from this point downward 
were very sparse, there being only one of note, which came off at a depth of 
about 20 inches and pursued an obliquely downward course for a distance of 
about 2.5 feet. Along the whole course of the root below the first 21 inches, 
very fine, pearly-white, hairlike branches were found in abundance. Par- 
ticularly was this true from 21 to 55 inches deep. In this region a large soil 
fissure occurred, and these pearly-white branches, oftentimes arising in pairs, 
were matted and profusely branched, extending horizontally from the main 
root to a distance of 10 inches in some cases. The matting of these small 
branches in the fissure was probably due to the excess of water which came 
down from the surface. Below this fissure these branches continued, but 
much fewer in number, to a depth of 60 inches, where a very moist, easily 
penetrable stratum of soil was encountered. In this stratum the small 
branches again occurred in great numbers, being sometimes 6 to 8 inches long 
and as many as 25 per linear foot. Here, too, they often occurred in pairs. 
The root is light brown in color. It has a fleshy cortex and a tough, fibrous 
stele; when broken a very small amount of latex exuded. 

Compared with this root type, that of the half-gravel-slide, with its shal- 
lower but more widely spreading and much more profusely branched roots, 
stands out in marked contrast. It shows a profoimd series of modifications in 
adapting itself to this very different habitat. This is evident from a com- 
parison of figures 57 and 58, notwithstanding the fact that the half-gravel- 
slide plant here illustrated was exceptionally deep-rooted, and from the follow- 
ing description, which is typical of one of several other half-gravel-slide 
plants examined. 

This was a large plant, with a tap an inch in diameter, which divided at a 
depth of 1 inch into two nearly equal parts. The one ran up the slope in a 
horizontal position and at a depth of only 4 to 10 inches. It soon broke up 
into many smaller branches, which diverged in all but a downward direction 
and extended to distances of 3 to 4 feet, and mostly only a few inches below 
the soil surface. The second descended vertically, but gave off 2 branches 5 
and 8 mm. in diameter, respectively, at a depth of 2 and 3 inches, while the 
tap was now only 5 mm. in diameter. The largest of these branches ran off 
at a depth of only 8 inches to a lateral distance of 5 feet. At 16 inches from 
the base of the plant it broke up into numerous equal-sized branches which 
rebranched profusely. None of these reached a depth greater than 14 inches, 
while many of the obHquely upward running branches filled the soil to near 
the surface with great masses of finely branched rootlets. The tap tapered 
very gradually and after branching considerably, mostly with vertically 
descending or oblique or even horizontal branches, reached a depth of 58 
inches. The spread of the deeper branches on either side of the tap was only 
about 12 inches. Tiny, fragile, glistening-white mats of branches form a 
network of rootlets 2 to 8 inches long, surrounding all but the larger parts of 
the older laterals. Four plants were examined. 




Fig. 57. — Euphorbia montana from 
the plains. 



Fig. 



58. — Euphorbia montana from the 
half-gravel-slide. 



ECADS. 



ROOT MODIFICATIONS OF POLYDEMICS. 

Of the 10 polydemic species here described, each growing in at least 
two different habitats, 7 show very striking changes in their root habits, 
2 made practically no change, while 1 exhibited only moderate differ- 
ences of root development. 

Bouteloua, Stipa, and Chrysopsis growing in the plains soil were all 
deep-rooted species, the latter indeed reaching a depth of over 13 feet. 
In the sandhills they all conformed to the typical root habit of most 
plants of this habitat. None of them reached more than half their 
former depth, but the shallow lateral root development and lateral 
spread were much more pronounced, especially in Chrysopsis. On the 
other hand, the more stable species as regards root development, 
Allionia linearis and Ahronia fragrans, when growing in these same 
habitats, were only sUghtly or not at all modified. The former showed 
practically no difference in the sandhill and plains ecads; the latter 
had a tendency to develop more and longer branches in the sand. 
These, with Kwleria, which was examined under four distinct sets of 
conditions in widely separated areas and found to be practically the 
same in all, form a very conservative group as regards root type, 
although, as shown by E. Clements (1905), the aerial parts of some are 
very plastic. This forms a unique problem for further investigation. 

Euphorbia and Yucca, while conforming to the plains root habit 
in having a rather deep and widely spreading absorbing system, were 
found to be profoundly modified when grown in the half-gravel. In 
the former the depth of penetration was always much less, while the 
branching, like that of Yucca, was much more pronounced. 

Of the three species, Smilacina stellata, Elymus triticoides, and 
Chamoenerium angustifolium, examined in gravel-slide or half-gravel- 
slide and in the forest, all took on the typical root habit characteristic 
of the several habitats respectively. All had fewer branches, a smaller 
lateral spread, and a lesser depth of penetration in the forest than in the 
open. The rather great root depth of Chamcenerium in the Douglas 
fir forest is an exception. This is somewhat puzzling and seems to be 
quite contrary to the usual rule of root development. 



122 



THE ECOLOGICAL RELATIONS OF ROOTS. 



X. SUMMARY. 

This study was undertaken to determine the root habits of dominant 
and subdominant plants growing under a wide range of cHmatic and 
edaphic conditions; to find the root relations of the plant communities 
as units of vegetation; and to determine the root distribution and root 
competition of the individual species in their relation to other species 
in the community. Other aims were to determine the relation between 
the root habits of plants in various communities and their successional 
sequence, and to obtain a more definite knowledge of the indicator 
value and the significance of various species used in classifying lands 
for grazing or for agriculture, as well as to aid the forester in selecting 
sites for afforestation or reforestation. 

The investigation extended over four years, during which time 
more than 1,150 individual root systems of about 140 species of shrubs, 
grasses, and other herbs were excavated and studied. These species 
were members of 8 different plant communities widely separated geo- 
graphically and growing under distinctly different climatic or edaphic 
conditions. The communities studied were the prairies of eastern 
Nebraska, the chaparral of southeastern Nebraska, prairies of south- 
eastern Washington, the plains association and sandhill subclimax of 
Colorado, and the gravel-slide, half -gravel-slide, and forest communities 
of the Rocky Mountains of Colorado. 

The trench or pit method was used in excavating the roots, the root 
system being removed with appropriate apparatus from the face of the 
trench. In case of roots extending to depths beyond 15 feet, two 
working levels were maintained. The quadrat-bisect method has been 
employed to show the root systems of communities in place and thus 
exhibit their interrelations in detail. 

The root systems of 33 species were examined in the prairies of 
eastern Nebraska; 13 of these were grasses. All of the dicotyledonous 
plants were found to extend well beyond a depth of 2 feet, while the 
roots of 6 of the grasses were confined to the surface 2 feet of soil. 
Four of the grasses and 5 other species were found to penetrate well 
below the second foot of soil, but seldom deeper than 5 feet. More 
than half of the plants studied, including three dominant grasses, have 
roots which reach depths greater than 5 feet — indeed, most of them 
penetrate to distances of from 7 to 9 feet and a few to a maximum 
depth of from 13 to 20 feet. 

These prairie species grow under a mean annual precipitation 
of about 28 inches, over 20 inches of which fall during the growing- 
season. The soil is of the type commonly called loess, much of which, 
however, is confounded with glacial drift. Soil-moisture determina- 
tions for more than two seasons show that the water-content is some- 
times reduced to such a degree that no moisture to a depth of 4 or 5 



SUMMARY. 



123 



feet is available for plant gTo\vi:h. The wind, prevailingly from the 
south or southeast during the growing season, averages about 70 
miles per day over the vegetation. The mean summer temperatures 
are high (70° to 75° F.), while the mean humidity is often low (58 to 70 
per cent). The average daily evaporation throughout the growing 
season is about 21 c.c. 

The plants of the prairie community, in response to these en\dron- 
mental conditions, have developed very efficient widely spreading and 
deeply penetrating root systems. The prairie community root habit 
emphasizes depth of penetration and widely spreading, deep laterals 
much more than the desert community (Cannon, 1911, 1913; Markle, 
1917) . The roots of prairie plants are grouped into more or less definite 
absorbing layers, many of the deeper-rooted species ha\dng few or 
no absorbing roots in the first few feet of soil. The layering of the 
roots reduces competition and permits the growth of a larger number 
of species. 

The root systems and mechanism of invasion of several dominant 
shrubs of the chaparral community, which occupies the tension zone 
between forest and prairie, were examined. All are supphed with 
splendid absorbing systems which are somewhat variable as to depth 
(5.5 to 21 feet), but all of which are deep-seated. In addition, they 
all have excellent methods of vegetative propagation. Their presence 
indicates a soil richer in humus and higher in water-content than 
that of adjacent grassland areas. These differences, as well as a lower 
evaporating power of the air, are brought about by the reactions of 
the shrubs. These consist of the accumulation of wind-blo\\Ti snow and 
plant debris, a lowering of the fight values and a consequent disap- 
pearance of most or all of the prairie species, obstruction to wind 
movement, lower temperatures and higher humidity, in part due to 
shade, and a less rapid oxidation of the humus materials. 

The root systems of 18 dominant and sub dominant species were 
examined in the prairies of southeastern Washington. These prairies 
represent an extreme westward extension of the great grassland 
formation hing east of the Rocky ]Mountains. They are characterized 
by the absence of late-maturing grasses and in general by an earfier 
seasonal development. Three of the four dominant grasses have root 
sj^stems confined to the surface 18 inches of soil. This is correlated 
with early seasonal growth and maturity. The fourth, hke the other 
herbaceous species, has a deep, widely spreading root system. All of 
these reached depths from 4 to 6 feet, w^hile some penetrated even 
deeper. However, the plants of the prairie community are not so 
deeply rooted as those of the prairies of eastern Nebraska. This may 
be correlated with the en\'ironment. 

These prairie species grow under a mean annual precipitation of 
21 inches, only about 7 inches of which fall diuring the gro\\dng season. 
But the silt-loam soil is extremely retentive of water, so that the 



124 



THE ECOLOGICAL RELATIONS OF ROOTS. 



precipitation, which occurs so gently that there is little run-off, is held 
as in a great reservoir that is rather thoroughly emptied during the 
following summer. Soil-moisture determinations for two seasons 
show that the water-content is sometimes reduced to the non-available 
point for plant growth, especially on exposed slopes, even to a depth 
of 3 or 4 feet. The wind, prevailingly from some southerly direction, 
averages about 96 miles a day just above the vegetation on south 
slopes. The air temperatures, which may reach 90"^ to 100° F., show 
a mean daily range varying from 25° F. in April and May to 38° F. 
in July and August. The mean daily soil temperatures at a depth of 
3 inches vary from 40° to 50° F. in May to from 66° to 92° F. in August, 
depending upon the slope. The humidity, normally low, frequently 
falls to 10 or 15 per cent during afternoons. The average daily evapora- 
tion throughout the growing season is about 29 c.c. on southerly slopes 
and 20 c.c. on northerly ones. 

Root penetration and the amount of branching were found in both 
the prairies of Washington and those of Nebraska to be profoundly 
affected by hard soil. Both the root length and the amount of branch- 
ing of a prairie species growing under the two conditions of soil texture 
were found to be more pronounced in the less compact soil. Plants 
growing in the mellow loess soil of eastern Nebraska are deeper-rooted 
than those growing in glacial drift soils. Aeration may be an impor- 
tant factor in effecting these modifications. Roots show a marked 
increase in their output of branches upon leaving compact soil and 
entering earthworm burrows or small crevices. 

The root systems of 28 important plains species were examined near 
Colorado Springs, Colorado. They were found to have deeply pene- 
trating and widely spreading roots. The lateral spreading of roots in 
the surface soil at depths of from 2 to 12 inches is much more pro- 
nounced than in either of the preceding prairie communities. Only 
four plants, including two cacti, had their root systems confined almost 
entirely to the surface 2 feet of soil. One-third of the plains species 
penetrated to depths greater than 2 feet but seldom beyond 5 feet. 
This group includes such dominant grasses as Bouteloua gracilis and 
Aristida purpurea. Nearly 60 per cent of the plains species had roots 
extending to depths greater than 5 feet. The roots of several plants 
reached depths of 10 to 12 feet. Although as a community plains 
plants are deeper-rooted than those of the prairies, no roots were found 
at a depth greater than 13 feet, as was the case with several prairie 
species in Nebraska. It is probable that water never penetrates more 
deeply than 13 feet in the plains. 

Like the plants of prairies and sandhills, the roots of plains species 
show a wide range in type from those with the tap as the principal 
feature or with the laterals placed near the surface and especially well 
branched, to roots with both tap and laterals well developed. How- 



SUMMARY. 



125 



ever, the latter or generalized type is the most conimon. Notwith- 
standing these individual differences, each community, viewed as a 
whole, has as its own root habit the one best fitted to the particular 
emironment. 

These plains species grow under a mean annual precipitation of 
about 15 inches, 12 inches of which fall during the groTvdng season. 
The soil is a sandy loam so compacted that ordinarily it can be removed 
only with a pick. Because of the great degree of compactness, run-off 
after rains is high. Moisture determinations during 1918 show that 
the soil to a depth of 7 feet is rather uniformly dry and that at certain 
periods during the summer no water is available for plant growth to 
a depth of 5 feet. The wind movement over the plants averages about 
120 miles per day. The daily fluctuation of temperature among the 
plants is usually about 35° to 40° F., the air reaching a maximum of 
90° to 95° F. Soil temperatures among the roots at a depth of 4 
inches ranged daily from 60° or 70° F. to 90° or 95° F. The average 
daily evaporation throughout the groT;ving-season is about 48 c.c. 

The well-developed system of shallow, widely spreading laterals is 
undoubtedly a response to the moisture in the surface soils resulting 
from frequent light summer showers. These surface roots are especially 
well developed in cacti. Those of Yucca are remarkable for their great 
lateral extent, often reaching a distance of more than 30 feet on every 
side of the plant. Bisects show that root competition among plains 
plants, especially in the first 2 feet of soil, is very severe. 

The root habits of 19 important sandhill species were studied in 
eastern Colorado, about 40 miles southeast of Colorado Springs; 8 of 
these have roots which are entirely or nearly confined to the surface 2 
feet of soil. The deeper-rooted species, none of which were traced 
below 11 feet because of the ca\TQg sand, practically all show a striking 
profusion of long, widely spreading laterals in this surface-soil stratum. 
The lower parts of deeply penetrating roots are often much more 
poorly branched. Several deep-rooted plains species under sandhill 
conditions develop only shallow roots. 

General cUmatic conditions are almost identical with those described 
for the plains; hence differences in root habit must be attributed to 
edaphic causes. Precipitated moisture is readily absorbed and there 
is practically no run-off, even in the heaviest showers. During dry 
weather the surface sand forms a very efficient means of retarding 
evaporation. Moisture determinations and observations show that 
the surface 2 or 3 feet of soil often is moister than the deeper strata, 
and thus offers a logical explanation for the typically shallow but 
widely spreading sandhill root habit. Several plants have in addition 
deeply penetrating roots; hence the deeper soils must receive moisture, 
perhaps during wet phases of the cHmatic cycle. Ipomoea leptophylla 
has the most extensive root development of any species examined in 
the eight communities studied. 



126 



THE ECOLOGICAL RELATIONS OF ROOTS. 



In the Rocky Mountains, adjoining the plains at Colorado Springs, 
the root systems of plants were studied in the gravel-slide, half -gravel- 
slide, and forest communities respectively. These communities form 
a developmental series. All plants of the gravel-shde community are 
similar in possessing root systems adapted to secure moisture and 
nutrients from the surface foot, and few extend beyond a depth of 18 
to, 24 inches. They have a remarkably wide lateral spread, combined 
with a most profuse system of branching. 

The surface soil consists of coarse, angular rock fragments, which 
are from over an inch to only a few millimeters in diameter. Below 
this lie a few inches of coarse gravel and sand which gradually give 
way, at a depth of 1 to 2.5 feet, to poorly disintegrated granite. The 
precipitation falls largely as frequent suromer showers which seldom 
wet the soil below 18 to 24 inches. The surface gravel serves efficiently, 
both in preventing run-off and in retarding evaporation. Moisture 
determinations show that the soil moisture, while at no time high, is 
rather equally distributed throughout the surface 18 inches of soil. 
Plants are so sparsely spaced that relatively little water is removed by 
transpiration. Wind movement averaged 103 miles daily. The aver- 
age daily evaporation throughout the summer was 34 c.c. The com- 
munity root habit is clearly a response to the environment. The 
development of strong laterals, characteristically up the slope, seems 
to be due in part to a response to the soil slipping down the mountains. 
Root competition on the gravel-slide is not at all severe. 

An examination of roots in the half -gravel-slide revealed quite a dif- 
ferent type. In addition to the shallow, widely spreading root habit, 
most species had supplementary deep roots which penetrated from 
2 to 4 feet. The soil of the former gravel-slide has now become deeper 
and richer in humus and much more densely populated. More than 
half of the surface is occupied by plants, dominant among which are 
certain grasses, while all of the soil to a depth of 3 feet is occupied by 
roots. The surface gravel has become more disintegrated and com- 
pacted and is less efficient in preventing evaporation. Because of 
competition for light, and especially for water, most gravel-slide 
plants have disappeared. Although wind movement is somewhat less 
and evaporation considerably lower than in the former barer habitat, 
the water-loss by transpiration from the denser plant population is 
higher. During periods of drought plants must rely largely upon 
the water in the deeper soil. More favorable deep-soil conditions, 
together with intense shallow-root competition, are factors largely 
determining root depth. 

Through a transition chaparral stage, the half-gravel-slide gives 
way to forest. Pinus ponderosa, Pseudotsuga mucronata^ or Picea engel- 
manni is the dominant, depending upon the amount of humus and 
moisture. The 19 herbs and shrubs examined on the forest floor were 



SUMMARY. 



127 



relatively shallow-rooted. Almost without exception the major por- 
tion of the absorbing system lies within the surface 18 inches. A few 
reUcts from earlier successional stages had roots which penetrated 
somewhat deeper. ]\Iany of the roots of tree dominants were found 
to be superficial. 

The water-content of the soil offers a logical explanation for this 
community root habit. The duff retards run-off as well as evapora- 
tion. The trees and shrubs reduce wind movement, lower the tem- 
perature and evaporation, and consequently greatly reduce water- 
losses by transpiration. Series of moisture determinations show that 
the surface 6 inches of soil usually have more available water than the 
under hing 6 inches, while the third foot is usually much drier than the 
second. This may be accounted for by the frequent summer showers. 
Not only are the herbs and shrubs of the forest floor shallow-rooted, 
but also as a rule the root systems are less extensive in proportion to 
the transpiring area than is the case with the half-gravel-sUde and the 
gravel-sHde plants. 

The root systems of several species growing in two or more dis- 
tinctly different habitats were studied. It was found that in most 
cases the root ijipe conformed to the community root habit in which the 
species was excavated. Thus profound differences, both in root depth 
and position and number of branches, were determined for most 
ecads. In general root position conformed strikingly with the distribu- 
tion of soil moisture. A few stable species were found which showed 
little or no variation of the root system when grown under different 
environmental conditions. 

The general characters of the root systems of a species are often as 
marked and distinctive as are the above-ground vegetative characters. 
But the root systems of different species of the same genus, while often 
somewhat similar, may be of entirely different tv^pes. 

A knowledge of the position and competition of roots is indispen- 
sable in explaining the phenomena of succession. Since root position 
so clearly reflects the moisture conditions of the soil, especially when 
interpreted in its community relations, a study of the root habits of 
plants greatly increases our knowledge of the value of various plants 
in indicating lands of agricultiu-al or non-agricultiu-al value. A knowl- 
edge of root habits is of further value in sohing the problems of the 
competition of range species and the improvement of the range. 
Finally, root systems indicate the distribution of soil moisture in 
various habitats, and thus should aid the forester in selecting sites for 
reforestation or afforestation. 



128 



THE ECOLOGICAL RELATIONS OF ROOTS. 



BIBLIOGRAPHY. 



Alway, F. J,, et al. 1916, The loess soils of the Nebraska portion of the transition region. 

Soil Science 1 : Nos. 3, 4, 5. 
Bates, C. G. 1910. Experiments in sandhill planting. Proc. Soc. Am. For. 5: 59. 
Caijnon, W. a. 1911. The root systems of desert plants. Carnegie Inst. Wash. Pub. No. 131. 

. 1912. Some featm^es of the root systems of desert plants. Pop. Sci. Mon. 41: 90. 

. 1913. Botanical featvires of the Algerian Sahara. Carnegie Inst. Wash. Pub. No. 178. 

. 1918. The evaluation of the soU-temperature factor in root growth. Plant World 

21:64. 

and E. E. Free. 1917. The ecological significance of soil aeration. Science 45: 178. 

Clements, E. S. 1905. The relation of leaf structure to physical factors. Trans. Am. Mic. 
Soc. 26: 19. 

Clements, F. E. 1904. Formation and succession herbaria. Univ. Nebr. Studies 4: 329. 
. 1905. Research methods in ecology. 

. 1916. Plant succession. Carnegie Inst. Wash. Pub. No. 242. 

KoBSTiAN, C. F. 1917. The indicator significance of native vegetation in the determination 
of forest sites. Plant World 20: 267. 

LovELAND, G. A. 1912. Summary of climatological data for the United States, Sec. 37, South- 
ern Nebraska. 

Markle, M. S. 1917. Root systems of certain desert plants. Bot. Gaz. 64: 177. 

Pool, R. J. 1914. A study of the vegetation of the sandhills of Nebraska. Minn. Bot. Studies 

4: 189:-> 

Pound, R., and F. E^ Clements. 1900. The phytogeography of Nebraska. 
Preston, C. E. 1900. Observations on the root systems of certain cactacese. Bot. Gaz. 
30: 348. 

RoBBiNS, W. W. 1917. The botany of crop plants. 

Sampson, A. W. 1914 Natural revegetation of range lands based upon growth requirements 

and life history of the vegetation. Jom*. Agr. Res. 3: 39. 
• . 1917. Important range plants; their life history and forage value. U. S. Dept. 

Agr. BuU. 545. 

Shants, H. L. 1911. Natural vegetation as an indicator of the capabilities of land for crop 
production in the Great Plains area. U. S. Dept. Agr., Bur. PI. Ind. Bull. 201. 

Ten Eyck, A. M. 1904. The roots of plants. Kans. Agr. Exp. Sta. Bull. 127. 

Thornbeb, J. J. 1901. The prairie-grass formation in region I. Rep. Bot. Surv. Nebr. 5:29. 

Weaver, J. E. 1915. A study of the root systems of prairie plants of southeastern Washington. 
Plant World 14:227. 

— . 1917. A study of the vegetation of southeastern Washington and adjacent Idaho. 

Univ. Nebr. Studies 17: 1. 
and A. F. Thiel. 1917. Ecological studies in the tension zone between prairie and 

woodland. Rep. Bot. Surv. Nebr., n. s. 1: 1. 



WEAVER 



PLATE 1 




A. One end of a trench used in excavating root systems. 

B. Distichlis spicata, showing the long rhizomes and shallow roots. 



A. An eastern Nebraska prairie in June. Erigeron ramosus and Meriolix serrulata in the 

foreground and bushy Psoralea tenuiflora in the background. 

B. An area dominated by Agrostis hiemalis with Allium mutabile, Achillea millefolium, and 

Stipa spartea. 



WEAVER 

A 



B 



PLATE 3 

c 




A. Androjpogon furcatus. B. Andropogon scoparius. C. Andropogon nutans. 



WEAVER 



PLATE 4 




A. Pafucum virgatum, showing rhizomes, coarse roots, and complete single root. 

B. Stipa s parte a. 



WEAVER 



PLATE 6 




A. Sporoholus longifolius. 

B. Bouteloua gracilis, excavated near the quadrat shown in figure 1. 



Silphium laciniatum, roots partially excavated. 
Amorpha canescens, showing wide lateral spread. 



PLATE 8 




A. Kuhnia glutinosa, roots partially excavated. 

B. Glycyrrhiza lepidota; a' is a continuation of a. 



WEAVER 



PLATE 9 





A. Psoralea tenuiflora, the tap-root decayed. 

B. Psoralea argophylla, showing entu'e root in center. 



WEAVER 



PLATE 1 




A. Bro.uneria pallida. B. Ly godesmia juncea, iniv^o seQiioris. C. Lespedeza capitata. 



WEAVER 



PLATE 12 




A. Ceo.nothus ovatus; root of a thirteen-year-old plant; a' is a continuation of a. 

B. Amorpha canescens; a' is a continuation of a. 



WEAVER 



PLATE 13 































\ \ 
















H 






\ 








f 

1 


t 

i 








1 









A 




A. Symphoricarpos vulgaris, showing fine network in surface soil. ^ 

B. Rhizomes and runners of SijmpJwricarpos vulgaris; the horizontal 

ground-line. 



WEAVER 



PLATE 14 




B 



A. Rhus glabra, a portion of the root network with ascending rootlet a. 

B. Rhus glabra, with ascending rootlets. 





A. Conjhis amcricana, the roots shown in two sections. 

B. Rosa orkausana, the roots shown in two sections. 

C. ConjJus amcricana, rhizomes and roots. 



A. Rosa arkansana, showing method of propagation. 

B. Rhus glabra invading subdimax prairie. 



WEAVER 



PLATE 17 




A. Yucca glauca, showing the multicipital stems and rhizome habit. 

B. Praii'ie of southeastern Washington. 

C. Meter quadrat in prairie, showing Balsamorhiza, Festuca, Lithospermum^,£Lnd Hieracium. 



WEAVER 



PLATE 18 




A. The plains association near Colorado Springs, showing Aristida purpurea 
bunches in Bouteloua gracilis tui'f. 

B. Psoralea tenuiflora in two sections. C. Yucca glauca. 



WEAVER 



PLATE 19 




WEAVER 



PLATE 20 




WEAVER 



PLATE 21 




A. A sandhill community, showing Redfieldia, Petalostemon villosus, Psoralea lanceolata^ 

and Chrysospis villosa, with a socies of Eriogonum microthecum at the left. 

B. Roots and rhizomes of Calamovilfa longifolia. 



WEAVER 



PLATE 23 




A. Consocies of Aletes acaulis on the gi-avel-slide, with Krynitzkia virgata in the foreground. 

B. Quadrat on the gravel-slide, showing detail of surface. 



WEAVER 



PLATE 24 




A. Aletes acaulis. 

B. Network of fine rootlets of Aletes. 



C. Smilacina stellata. 

D. Thlaspi alpestre. 




A. Half-gravel-slide, showing Elymus triticoides and the large bare intervals. 
B. Elymus triticoides. C. Ruhus deliciosus. 



WEAVER 



PLATE 27 




B 



A. Picea engelmanni consociation, showing the forest floor. 

B. Quadrat in the same spruce forest, showing Haplopappus parryi, Fragaria virginiana, 

Thalictrum fendleri, etc. 



WEAVER 



PLATE 28 




A, ArctostapJiyJos uva-ursi. showing a poniou oi the root system. 

B. Ribts Jacustrt. seven years old. 



WEAVER PLATE 29 




PLATE 30 

WEAVER 




A. Opulaster opulifolius, twelve years old. 

B. Chrysopsis villosa from the sandhills. 



